PYTHON 3

Pocket Primer

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PYTHON 3

Pocket Primer

James R. Parker

MERCURY LEARNING AND INFORMATION

Dulles, Virginia
Boston, Massachusetts
New Delhi

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James R. Parker. Python 3 Pocket Primer.
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CONTENTS

Preface

Chapter 1: Computers and Programming

Executing Python

Rock-Paper-Scissors

Solving the Rock-Paper-Scissors Problem

Variables and Values – Experimenting with the Graphical User Interface

Exchanging Information with the Computer

Example 1: Draw a Circle Using Characters

Strings, Integers, and Real Numbers

Number Bases

Example 2: Compute the Circumference of Any Circle (Input)

IF Statements

Else

Documentation

Rock-Paper-Scissors Again

Types Are Dynamic

Chapter 2: Repetition

The WHILE Statement

Modules and Random Numbers

Counting Loops

Prime or Non-Prime

Exiting from a Loop

Else

Exceptions and Errors

Chapter 3: Sequences: Strings, Tuples, and Lists

Strings

Comparing Strings

Problem: Does a City Name, Entered at the Console, Come before or after the Name Denver?

Slicing – Extracting Parts of Strings

Problem: Identify a “print” Statement in a String

Editing Strings

Problem: Create a JPEG File Name from a Basic String

Problem: Change the Suffix of a File Name

Problem: Reverse the Order of Characters in a String

Problem: Is a Given File Name That of a Python Program?

String Methods

Spanning Multiple Lines

For Loops Using Strings

The Type Bytes

Tuples

Tuples in For Loops

Problem: Print the Number of Neutrons in an Atomic Nucleus

Membership

Problem: What Even Numbers Less than or Equal to 100 Are Also Perfect Squares?

Delete

Problem: Delete the Element Lithium from the Tuple atoms, along with Its Atomic Number

Update

Problem: Change the Entry for Lithium to an Entry for Oxygen

Tuple Assignment

Built-In Functions for Tuples

Lists

Problem: Compute the Average (Mean) of a List of Numbers

Editing Lists

Insert

Append

Extend

Remove

Index

Sort

Reverse

Count

List Comprehension

Lists and Tuples

Exceptions

Problem: Delete a Specified Element from a List

Set Types

Example: Craps

Chapter 4: Functions

Function Execution

Problem: Write a Function to Calculate the Square Root of Its Parameter.

Parameters

Default Parameters

None

Example: The Game of Sticks

Scope

Variable Parameter Lists

Variables as Functions

Example: Find the Maximum Value of a Function

Functions as Return Values

Recursion

Creating a Python Module

Chapter 5: Files: Input and Output

Problem: Read a Number from the Keyboard and Divide It by 2

Using Files in Python

Open a File

File Not Found Exceptions

Reading from Files

End of File

Common File Input Operations

CSV Files

Problem: Print the Names of Planets Having Fewer than Ten Moons

The With Statement

Writing to Files

Example: Write a Table of Squares to a File

Appending Data to a File

Example: Append Another 20 Squares to the Table of Squares File

Chapter 6: Classes

The Python Class – Syntax and Semantics

A Really Simple Class

Encapsulation

Example: A Deck of Cards

Cat-a-Pult

Basic Design

Detailed Design

Subclasses and Inheritance

Non-Trivial Example: Objects in a Video Game

Duck Typing

Chapter 7: Graphics, Media, and Interfaces

Installing Pygame

Essentials: The Graphics Window and Colors

Simple Static Drawing

Pixel-Level Graphics

Example: Create a Page of Notepaper

Example: Creating a Color Gradient

Lines and Curves

Example: Notepaper Again

Polygons

Blitting

Drawing Text

Transparent Colors

Images

Pixels

Example: Thresholding

Interaction

Time

Events

The Mouse

Button

Stretchy Lines

The Keyboard

Sound

A Game

The Lander

Movement

The Background

Sound

Landing

Improvements

Conclusion

Chapter 8: Handling Data

Dictionaries

Functions for Dictionaries

Dictionaries and Loops

Arrays

Formatted Text, Formatted I/O

Example: NASA Meteorite Landing Data

Both Strings Will Be Left Aligned

Advanced Data Files

Binary Files

Example: Create a File of Integers

The Struct Module

Example: A Video Game High Score File

Random Access

Example: Maintaining the High Score File in Order

Chapter 9: Communication Using Python

Email

Example: Send an Email

Reading Email

Example: Display the Subject Headers for Emails in Inbox

Communication Between Processes

Example: A Server That Calculates Squares

Index

PREFACE

This book is an effort to give a programmer sufficient knowledge of Python 3 to be able to work on their own projects. It is based on a much longer book that was intended for beginning programmers, and so most of the introductory material and basic computer science has been removed.

What remains is, first, a lot of code. Programming is something that must be practiced, and this book provides a lot of examples that are intended to inspire the explanations of programming language structures that otherwise lack context. Many of the examples are games or portions of games. That’s because most of the audience are game players of one kind or another and can understand the examples. The code that implements the game is motivated by that understanding, although there are always many different actual programs that can be written to solve any one problem.

The example code was compiled on a PC running Windows 10, using Python 3.4 and the PyCharm GUI. Working code was copied directly into the manuscript and so should always be functional, but however hard we try, sometimes errors creep in during production. Please let me know if you find one.

There are a couple of unique features of this short book. One is the chapter on PyGame, which allows a programmer to handle graphics, control mouse and keyboard interaction, and play sounds and videos. The large example for that chapter is a Lunar Lander game.

Another feature is the chapter on communication, which makes use of one of Python’s best features: a collection of modules for sending and receiving Email, communicating between computers, and working with Twitter and Web pages.

The disc that accompanies this book contains all of the code examples as complete working programs (also available for downloading from the publisher). This means that there is no need to type them in so they can be executed and perhaps modified or expanded. The disc also contains all of the figures in the book at their original size. Some of these are used as data for the programs, so it’s good to have them.

There is a large code base in both Python 2.7 and Python 3, and one must take care when installing and using any module that it is compatible with the version of Python that has been installed—the two are incompatible.

Python 2 vs. Python 3

Here are the critical differences between the two versions of Python.

division

In Python 2, dividing two integers results in an integer: 3/2=1. In Python 3, ‘/’ is floating point division, so 3/2 is 1.5.

byte

Python 2 has no byte type.

xrange

Python 3 has no xrange function.

exception

Python 3 requires that when raising an exception, the exception argument must be enclosed in parentheses. For example:

raise IOError (“missing file”) # Py3

as opposed to

raise IOError, “missing_file” # Py2

print

In Python 2, print is a statement, but in Python 3 it is a function.

There are other minor differences. Also, some of the features of Python 2 that were considered to be mistakes were repaired in Python 3 (e.g., rounding, parsing user input using input()).

J. Parker
March 2017

CHAPTER 1

COMPUTERS AND PROGRAMMING

We are going to learn a language called Python. It was developed as a general-purpose programming language and is a good language for teaching because it makes a lot of things easy. Quite a few applications have been built using Python, such as the games Eve Online and Civilization IV, BitTorrent, and Dropbox to name only a few. It is a bit like a lot of other languages in use these days in terms of structure (syntax) but has some simplifying ideas that will be discussed in later chapters.

In order to use a programming language, there are some basic concepts and structures that need to be understood at a basic level, and if you already program in Java or C++ then you likely already know a lot of these. The book will teach you Python by example; coding examples will be introduced by stating a problem to be solved. The problems to be solved in this chapter include the game of rock-paper-scissors. These problems will be the motivation for learning more about either the Python language itself or about methods of solving problems. Any computer programs in this book will execute on a computer running any major operating system once the free Python language download has been installed.

Executing Python

Installing Python is not too difficult, and involves downloading the installer, running it, and perhaps configuring a few specific details. This process can be found in many places on the Internet, including http://docs.python-guide.org/en/latest/starting/installation/. Python works on nearly any system. Once installed there are a few variations that can be used with it, the simplest probably being the Python Graphical User Interface or GUI. If running Python on a Windows PC, look at the Start menu for Python and click; a link named “IDLE (Python GUI)” will be seen, as shown in Figure 1.1. Click on this and the user interface will open. Click the mouse in the GUI window so that you can start typing characters there.

FIGURE 1.1. Running Python using IDLE.

Python can be run interactively in the GUI window. The characters “>>>” are called a prompt, and indicate that Python is waiting for something to be typed at the keyboard. Anything typed here will be presumed to be a Python program, or at least part of one. As a demonstration, type “1” and press “Enter.” Python responds by printing “1.” Why? When “1” was typed it was a Python expression, something to be evaluated. The value of “1” is simply “1,” so that was the answer Python computed.

Now type “1+1.” Python responds with “2.” Python inputs what the user/programmer types, evaluates it as a mathematical (in Python form) expression, and prints the answer. This is not really programming yet, because a basic two-dollar calculator can do this, but it is certainly a start.

IDLE is good for many things, but eventually a more sophisticated environment will be needed, one that can indent automatically, detect some kinds of errors, allow programs to be run and debugged and saved as projects. This kind of system is called an integrated development environment, or IDE. There are many of these available for Python, some costing quite a lot and some freely downloadable. The code in this book has been compiled and tested using one called PyCharm, but most IDEs out there would be fine, and it is largely a matter of personal preference. Basic PyCharm is free and it has a bigger brother that costs a small amount.

An advantage of an IDE is that it is easy to type in a whole program, run it, find the errors, fix them, and run it again. This process is repeated until the program works as desired. Multiple parts of a large program can be saved as separate files and collected together by the IDE, and they can be worked on individually and tested together. And a good IDE uses color to indicate syntax features that Python understands and can show some kinds of error while the code is being entered.

Rock-Paper-Scissors

Although this game is used by children to settle disputes and make random decisions such as “who goes first,” it has been taken more seriously by adults. There are actually competitions where money is at stake. A televised contest in Las Vegas had a prize of $50,000. This game is not as trivial as it once was.

In this game each of two players selects one item from the list [rock, paper, scissors] in secret, and then both display their choice simultaneously. If both players selected the same item, then they try again. Otherwise, rock beats scissors, scissors beats paper, and paper beats rock. This contest can be repeated for a “best out of N” competition.

Both of these games form the first problem set, and serve as the motivation for learning the elements of the Python language.

Solving the Rock-Paper-Scissors Problem

The solution to this problem uses only basic language features. One solution to this problem is:

1)Select a random choice from the three items: rock, paper, or scissors. Save this choice in a variable named choice.

2)Ask the player for their choice. Use an integer value, where 1 = rock, 2 = paper, and 3 = scissors.

3)Read the player’s selection into a variable named player.

4)If player is equal to choice:

5)  Print the message “Tie. We’ll try again.”

6)  Repeat from step 1.

7)If player is equal to rock:

8) If choice is equal to scissors go to step 17

9) Else go to step 18.

10)If player is equal to paper:

11)  If choice is equal to scissors go to step 17

12)Else go to step 18.

13)  If player is equal to scissors:

14)If choice is equal to rock go to step 17

15)  Else go to step 18.

16)Print error message and terminate.

17)Print “computer wins” and terminate.

18)Print “You win” and terminate.

For each player selection, one of the alternate items will beat it and one will lose to it. Each choice is checked and the win/lose decision is made based on the known outcomes.

The solutions to both problems require similar language elements: a way to store a value (a variable), a way to execute specific parts of the program depending on the value of a variable or expression (an if statement), a way to read a value from the keyboard, a way to print a message on the screen, and a way to execute code repeatedly (a loop).

Variables and Values – Experimenting with the Graphical User Interface

A variable is a name that the programmer can define to represent some value, a number, or a text string generally. Not all strings or characters can be variable names. A variable cannot begin with a digit, for example, or with most non-alphabetic characters like “&” or “!,” although in some cases beginning with “_” is acceptable. A variable name can contain upper- or lowercase letters, digits, and “_.” Uppercase and lowercase are distinct, so the variables Hello and hello are different.

Programs often have variables named i or x. However, it is a good idea to select names that represent the kind of value that the variable is to contain, so as to communicate that meaning to another person, probably a programmer. For example, the value 3.1415926 should be stored in a variable named pi, because that’s the name everyone else gives to this value.

In the GUI, type pi = 3.1415926. Python responds with a prompt, which indicates that it is content with this statement, and that it has no value to print. If you now type pi, the response will be 3.1415926; the variable named pi that was just created now has a value.

In the syntax of Python, the name pi is a variable, the number 3.1415926 is a constant but also an expression, and the symbol = means assign to. In the precise domain of computer language, pi = 3.1415926 is an assignment statement and gives the variable named pi the specified value.

Continuing with this example, define a new variable named radius to be 10.0 using an assignment statement radius = 10.0. If you type radius and press “enter,” Python responds with 10.0. Finally, we know that the circumference of a circle is 2πr in math terms, or 2 times pi times the radius in English. Type 2*pi*radius into the Python GUI, and it responds with 62.831852, which is the correct answer. Now type circumference = 2*pi*radius, and Python assigns the value of the computation to the variable circumference.

Python defines a variable when it is given a value for the first time, and does not require a declaration. The type of the variable is defined at that moment too; that is, if a number is assigned to a name, then that name is expected to represent a number from then on. If a string is assigned to a name, then that name will be expected to be a string from then on. Trying to use a variable before it has been given a value and a type is an error. Attempting the calculation:

area = side*side

is not allowed unless there is a variable named side already defined at this point. The following is OK because it defines side first, and then in turn is used to define area:

side = 12.0
area = side*side

The two lines above are called statements in Python, and a statement usually ends at the end of the line (the “enter” key was pressed). This is a bit unusual in a computer language, and people who already know Java or C++ have some difficulty with this idea at first. In other computer languages statements are separated by semicolons, not by the end of the line. In fact, in most languages the indenting of lines in the program does not have any meaning except to the programmer. In Python indenting matters a great deal, as will be seen shortly.

The expressions we use in assignments can be pretty complicated, but they are really only things that we learned in high school and are the same, essentially, as in Java. We can add, subtract, multiply, and divide. Precedence rules for math operations are the same as for Java: multiplication and division are performed before addition and subtraction, otherwise evaluation is done left to right, so 6/3*2 is 4 (do the division first) as opposed to 1 (if the multiplication was done first). These are rules that should be familiar because people are taught to do arithmetic in this way. The symbol “**” means exponent or to the power of, so 2**3 is 2^3 which is 8, and this operator has a higher precedence than (i.e., is done before) the others. Parentheses can be used to specify the order of things. So, for example, (2+3)**2 is 25, because the expression within the parentheses is done first, then the exponent.

Exchanging Information with the Computer

When using most programming languages, it is necessary to design communication with the computer program. This goes two ways: the program will inform the user of things, such as the circumference of a circle given a specific radius, and the user may want to tell the program certain things, like the value of the radius for computing the circumference. We communicate with a program using text, which is to say characters typed into a keyboard. When a computer is presenting results, that text is often in the form of human language, messages as sentences. “The circumference is 62.831852” could be such a message. The sentence is actually composed by a programmer and has a number or collection of numbers embedded within it.

FIGURE 1.2. A Python GUI window showing the examples so far.

Java generally makes text I/O a little difficult, especially to and from files. Python allows a programmer to send a message to the screen, and hence to the user, using a print directive. This is the word print followed by a character string, which is often a set of characters in quotes. An example:

print ("The answer is yes.")

The parentheses are used to enclose everything that is to be printed; such a statement can print many strings if they are separated by commas. Numbers will be converted into strings for printing. So the following is correct:

print ("The circumference is ", 62.831852)

If a variable appears in the list following print, then the value of that variable will be printed, not the name of the variable. So:

print ("The circumference is ", circumference)

is also correct.

Example 1: Draw a Circle Using Characters

Assuming that it is desired to print a circle having a constant predefined radius, this can be done with a few print statements. The planning of the graphic itself (the circle) can be done using graph paper. Assuming that each character uses the same amount of space, a circle can be approximated using some skillfully placed “*” characters. Then print each row of characters using a print statement. A sample solution is:

print ("       ***        ")

print ("    *********     ")

print ("  *************   ")

print (" ***************  ")

print (" ***************  ")

print (" ***************  ")

print ("  *************   ")

print ("    *********     ")

print ("       ***        ")

Strings, Integers, and Real Numbers

A Python variable can hold either integers or real numbers (floats), but if a variable contains an integer then it is treated as an integer, and if it’s holding a floating-point number then it is treated as one of those. What’s the difference? First, there’s a difference in how they are printed out. If we make the assignment var = 1 and then print the value of var, it prints simply as 1. If we make the assignment var = 1.0 and then print var, it prints as 1.0. In both cases var is a real or floating point number and will be treated as such. A variable can be first one thing and then another; it will be the last type it was assigned. Typed variables as seen in Java are not available in Python.

Arithmetic differs between integers and reals, but the only time that difference is really apparent is when doing division. Integers are always whole, non-fractional numbers. If we divide 3 by 2, both 3 and 2 are integers and so the division must result in an integer: the result is 1. This is because there is exactly a single 2 in 3, or if you like, 2 goes into 3 just once, with a remainder of 1. There is a specific operator for doing integer division: “//.” So, 3//2 is equal to 1. The remainder part can’t be handled and is discarded, but it can be found separately using the “%” operator. For example, 8//5 is 1, and 8%5 is the remainder, 3.

Of course fractions work fine for real numbers, and will be printed as decimal fractions: 8.0/5.0 is 1.6, for example. What happens if we mix reals and integers? In those cases things get converted into real, but now things get more complicated because order can matter a great deal. The expression 7//2*2.0 does the division 7//2 first, which is 3, and then multiplies that by 2.0, yielding the result 6.0; the result of 8/3*3.0 would be 5.333. Mixing integers and reals is not a good idea, but if done then the expressions should use parentheses to specify how the expression should be evaluated.

A real can be used in place of an integer in most places, but the result will be real. Thus, 2.0 * 3 = 6.0, not 6, and 6.0//2 is 3.0, not 3. There are some exceptions. To convert an integer to a real, there is a special operation named float: float(3) yields 3.0. Of course it’s possible to simply multiply by 1.0, and the result will be float too. Converting float values to integers is more complicated, because of the fraction issue: what happens to the digits to the right of the decimal? The operation int will take a floating-point value and throw away the fraction. As a result, the value of int(3.5) will be 3. It’s normal in human calculations to round to the nearest integer, and the operation round(3.5) does that, resulting in 4.

Number Bases

In Python numbers are given in decimal (base 10) by default. However, if a numeric constant begins with “0o” (zero followed by the letter “o”), Python assumes it is base 8 (octal). The number 0o21, for example, is 21_8 = 17_10. A number that begins with “0x” is hexadecimal. 0x21 is 21_16 = 33_10. This applies only to integers.

There is a number base that is the most important, because it lies under all of the numbers on a computer. That would be base 2. All numbers on a modern digital computer are represented in base 2, or binary, in their internal representation. A binary number has only two digits, 0 and 1, and each represents a power of 2. Thus, 1101_2 is 1*2^3 + 1*2^2 + 0*2^1+ 1 = 8 + 4 + 1 = 13_10. In Python a binary number begins with “0b,” so the number 0b10101 represents 21_10.

Example 2: Compute the Circumference of Any Circle (Input)

When humans send information into a computer program, the data tends to be in the form of numbers, but humans use text as the means to communicate them. The Python code that was written to calculate the radius of a circle only did the calculation for a single radius: 10. That’s not as useful as a program that computes the circumference of any circle, and that would mean allowing the user to tell the program what radius to use. This should be easy to do, because it is something that is needed frequently. Frequently needed things should always be easy. In the case of sending a number into a program in Python, the word input can be used. For example:

radius = input ()

will accept a number from the keyboard, typed by the user, and will return it as a string of characters. This makes sense because the user typed it as a string of characters, but it can’t be used in a calculation in this form. To convert it into the internal form of a number, we must specifically ask for this to be done:

radius = input()
radius = float(radius)

will read a string into radius, then convert it into a floating-point (real) number and assign it to the variable radius again. This can be done all in one statement:

radius = float(input())

Now the variable radius can be used to calculate a circumference. This is a whole computer program that does a useful thing. If the value of radius was to be an integer, the code would read:

radius = int(input())

If the conversion to a number is not done, then Python will give an error message when the calculation is performed, as in the following:

Traceback (most recent call last):

File "<pyshell#13>", line 1, in <module>

circumference = 2*pi*radius

TypeError: can't multiply sequence by non-int of

type 'float'

This is pretty uninformative to a beginning programmer. What is a Traceback? What’s pyshell? There are clues as to what this means, though. The line of code at which the error occurs is given and the term TypeError is descriptive. This error means that something that can’t be multiplied (a string) was used in an expression involving a multiplication. That thing is the variable radius in this instance, because it was a text string and was not converted to a number.

Also note that int(input()) can present problems when the input string does not in fact contain an integer. If it is a floating-point number, this results in an error. The expression int(“3.14159”) could be interpreted as an attempt convert the string containing pi into an integer, and would have the value 3; in fact it is an error. The function int was passed a string and the string contained a float, not an int. This is something of a quirk of Python. It is better to convert input numbers into floats.

A working program for this would be:

print ("This program finds the circumference of a circle.")

radius = input ("Enter the radius of the circle: ")

radius = float(radius)

circ = radius *2.0 * 3.1415926

print ("The circumference is ", circ)

IF Statements

The word “if” indicates a standard conditional sentence in English. The condition in the first case is the phrase “if the light is red” (called in English the protaxis or antecedent) and the consequence to that is the phrase “then stop” (the apodosis or consequent). Terminology aside, the intent is clear to an English speaker: on the condition that the light is red, then the necessary action is that the driver is to stop their car. The action is conditional on the antecedent, which in Python will be called an expression or, more precisely, a logical expression, which has the value True or False.

The structure or syntax of this sort of thing in Python would be:

if the light is red:

stop

or more exactly:

if light == red:

# execute whatever code makes the car stop

This is a Python if statement.

In Python an if statement begins with the word if, followed by an expression that evaluates to True or False, followed by a colon (:), and then a series of statements that are executed if the expression is true. The names True and False are constants having the obvious meaning, and a variable that can take on these values is a logical or Boolean variable. The expression is the only tricky part. It can be a constant like True, or a variable that has a True or False value, or a relational expression (one that compares two things) or a logical combination of any of these—anything that has a result that is true or false.

if True:

# Constant

if flag:

# Logical variable

if a < b:

# relational expression

if a<b and c>d:

# logical combination

A logical expression can be any arithmetical expression using any of the following operators:

<

Less than

>

Greater than

<=

Less than or equal to

>=

Greater than or equal to

==

Equal to

!=

Not equal to

Logical combinations can be:

and

e.g.:

a==b and b==c

or

e.g.:

a==b or a==c

not

e.g.:

not (a == b)

# same as !=

The syntax is simple and yet allows a huge number of combinations. For example:

if p == q and not p ==z and not z == p:

if pi**2 < 12:

if (a**b)**(c-d)/3 <= z**3:

FIGURE 1.3. Syntax of an IF statement.

The consequent, or the actions to be taken if the logical expression is true, follows the colon on the following lines. The next statement is indented more than the if, and all statements that follow immediately that have the same indentation are a part of the consequent and are executed if the condition is true, otherwise none of them are. As an example, consider:

if a < b:

a = a + 1

b = b – 1

c = a – b

In this case the two statements following the “:” are indented four spaces past the if line. This tells Python that they are both part of the if statement, and that if the value of a is smaller than the value of b, then both of those statements will be executed. Python calls such a group of statements a suite. The assignment to the variable c is indented to the same level as the if, so it will be executed in any case and is not conditional.

The use of indentation to connect statements into groups is unusual in programming languages. Most languages in use pretty much ignore spaces and line breaks altogether, and use a statement separator such as a semicolon to demark statements. So, in the Java language the above code would look like this:

if (a<b) {

a = a + 1;

b = b – 1;

}

c = a – b;

The braces { . . . } enclose the suite, which would probably be called a block in Java or C++. Notice that this code is also indented, but in Java this means nothing to the computer. Indentation is used for clarity, so that someone reading the code later can see more clearly what is happening.

Semicolons are used in Python too, but much more rarely. If it is desired to place more than one statement on a single line, then semicolons can be used to separate them. The Python if statement under consideration here could be written as:

if a < b: a = a + 1; b = b 1

c = a - b

This is harder to comprehend quickly and is therefore less desirable. There are too many symbols all grouped together. A program that is easy to read is also easier to modify and maintain. Code is written for computers to execute, but also for humans to read.

There are some special assignment operators that can be used for incrementing and decrementing variables. In the above code the statement a = a + 1 could be written as a += 1, and b = b – 1 can be written as b –= 1. There is no real advantage to doing this, but other languages permit it so Python adopted it too. There is another syntax that can be used to simplify certain code in languages like Java and C, and that is the increment operator “++” and the decrement operator “—”: Python does not have these. However, an effect of the way that Python deals with variables and expressions is that “++x” is legal; so is “++++x.” The value is simply x. The expression “x++” is not correct.

Else

An if statement is a two-way or binary decision. If the expression is true, then the indicated statements are executed. If it is not true, then it is possible to execute a distinct set of statements. In one case the computer wins, and in the other the human wins. An else clause is what will allow this.

The else is not really a statement on its own, because it has to be preceded by an if, so it’s part of the if statement. It marks the part of the statement that is executed only when the condition in the if is false. It consists of the word else followed by a colon, followed by a suite (sequence of indented statements). So a trivial example is:

if True:

print ("The condition was true")

else:

print ("the condition was false")

Note the indentation. The else as a clause is not required to accomplish any specific programming goals, and can be implemented using another if. The code:

if a < b:

print ("a < b")

else:

print ("a >= b")

could also be written as:

if a < b:

print ("a < b")

if not (a<b):

print ("a >= b")

The else is expressive, efficient, and syntactically convenient. It is expressive because it represents a way that humans actually communicate. The word else means pretty much the same thing in Python as it does in English and Java.

Documentation

There are some problems with this program, but it does work. A large problem is that it always chooses the same number every time it is executed (that number being 7). This will be fixed later on. A less critical problem is that it is undocumented; that is, there are no instructions to a player concerning how to use the program and there is no description of how the program works that another programmer might use if modifying this code. This can be fixed by providing internal and external documentation.

External documentation is like a manual for the user. Most programs have such a thing, and even though this program is quite simple, some degree of documentation can be provided. In fact, it is brief enough that it could be printed whenever the program starts to run. For example:

print ("Rock-Paper_Scissors is a simple guessing game.")

print ("The computer will prompt you for your choice, ").

print ("which must be one of 'rock', 'paper', or 'scissors'")

print ("When you select a choice the computer will too (it ")

print ("will not cheat) and the winner is selected by three ")

print ("simple rules: rock beats scissors, paper beats ")

print ("rock, and scissors beat paper. It a tie happens")

print ("then you should play again.)

For many more sophisticated programs, such as PowerPoint, for example, the documentation is many pages and forms a small book. It would be distributed as a booklet along with the software or provided as a web site.

High-level languages like Python allow the programmer to add human language comments to the code that will be completely ignored by the computer, but that can be read by anyone looking at the code. These comments describe the action of the program, the meaning of the variables, details of computational methods used, and many other items of interest.

In Python a comment begins with the character “#” and ends at the end of the line. There are no rules for what can appear typed in a comment, but there are some guidelines developed through years of programming practice. A comment should not simply repeat what appears in the code; a comment should shed some light on an aspect of the program that might not be clear to everyone looking at it or document some of the history of the code, and it should be written in plain language. For instance:

# This program plays the game known as Rock-

# Paper-Scissors.

# Programmed by J. Parker Jan-2017

There is also something called a docstring that seems to do the same things as a comment, but covers multiple lines and is not really a comment. A docstring begins and ends with a triple quote:

print ("This code will execute")

"""

print ("This code is within a docstring")

"""

A docstring is actually a string, not a comment, but it behaves like a comment and can be used in that way. It can be especially useful for temporarily commenting out small sections of code while trying to find out where errors are. There are also programs that will collect the docstrings into a separate document that can be used as a description of the program. For that reason an intended use is to allow the programmer to explain the purpose of certain sections of code.

Rock-Paper-Scissors Again

With what is now known about Python, it is time to look at the rock-paper-scissors problem and see if it can be coded. It takes more steps, but it is really no more complicated that the guess-number program. The code is the same.

1)Select a choice from the three items: rock, paper, or scissors. Save this choice in a variable named choice.

A representation for the three items was decided upon when the solution was first described, where each choice was an integer. However, input reads strings, so it should be possible to avoid the conversion to numbers and use the strings directly.

choice = "paper" # Computer chooses paper.

2)Ask the player for their choice.

Print as a prompt message.

print ("Rock-paper-scissors: type in your choice: ")

3)Read the player’s selection into a variable named player.

Use input as we did before, but this time read it as a string and keep it that way. The player must type either “rock,” “paper,” or “scissors,” or else an error will be reported.

player = input ()

4)If player is equal to choice:

5)  Print the message “Game is a tie. Please try again.”

Strings can be compared against each other for equality, so this step is quite simple:

if player == choice:

print ("Game is a tie. Please try again.")

6)If player is equal to rock

7)  If choice is equal to scissors go to step 17

There will be no “go to step 17,” but that step simply says that the player wins. Just print that message here.

if player == "rock":

if choice == "scissors":

print ("Congratulations. You win.")

else:

print ("Sorry - computer wins.")

8)If player is equal to paper

9)If choice is equal to scissors go to step 17

if player == "paper":

if choice == "scissors":

print ("Sorry - computer wins.")

else:

print ("Congratulations. You win.")

10)If player is equal to scissors

11)If choice is equal to rock go to step 17

if player == "scissors":

if choice == "rock":

print ("Sorry - computer wins.")

else:

print ("Congratulations. You win.")

This code illustrates a new concept, if not a new language feature. It has if statements that are nested one within the other. Again, it’s not necessary to do this because non-nested statements can implement the same decision. For example:

Nested IFs

Non-nested IFs

if player == “scissors”:

if player == “scissors

and choice == “rock”

if choice == “rock”:

print (“Computer wins”)

print (“Computer wins.”)

if player == “scissors”

and choice != “rock”

else:

print (“You win”)

print (“You win.”)

Nested if statements seem more expressive, and communicate the flow of the program better to a human programmer than does the non-nested code.

There is another Python language element that can be used here. Looking at the code, there is no indication when the user makes an error. For example, if the user enters “ROCK” (i.e., uppercase), then it will not match any of the choices and the program will not indicate this. In fact it won’t print anything at all. What is really wanted is a sequence of if-else-if-else statements such as:

if player == "scissors":

if choice == "rock":

else:

if player == "rock":

if choice == paper:

else:

if player == "scissors":

## and so on …

Python has a special feature that implements this nesting of if and else: the elif. The elif construct combines an else and an if, and this reduces the amount of indenting that has to be done. The following code snippets do the same thing:

if a<b:

if a<b:

print ("a<b")

'print ("a<b")

elif a>b:

else:

print ("a>b")

'if (a>b):

else:

print ("a>b")

print ("a=b")

else:

print ("a=b")

If too many nested if-else statements exist, then the indenting gets to be too much, whereas the elif allows the same indent level and has the same meaning.

Types Are Dynamic

To programmers who only program using Python, it would seem odd that a particular variable could have only one type, as is the case in C++, and that it would have to be initially defined to have that type, but it is true. In Python the type associated with a variable can change. For example, consider the statements:

x = 10

# X is an integer

x = x*0.1

# X is floating point now

x = (x*10 == 10)

# X is Boolean

Some find this perfectly logical, and others find it confusing. The fact is that so long as the variable is used according to its current type, all will be well.

It is also true that even apparently simple Python types can be quite complex in terms of their implementation. The point is that the programmer rarely needs to know about the underlying details of types like integers. In many programming languages an integer is simply a one- or two-word number, and the languages build operations like “+” from the instruction set of the computer. If, for example, a one-word integer A is added to another one B, it can be done using a single computer instruction like ADD A, B. This is very fast at execution time.

Python was designed to be convenient for the programmer, not fast. An integer is actually a complex object that has attributes and operations. This will become clearer as more Python examples are written and understood, but as a simple case think about the way that C++ represents an integer. It is a 32-bit (4-byte) memory location, which is a fixed-size space in memory. The largest number that can be stored there is 2^32-1. Is that true in Python?

Here’s a program that will answer that question, although it uses more advanced features:

for i in range (0,65):

print (i, 2**i)

Even an especially long integer would be less than 65 bits. The fact is that this program runs successfully, and even rather quickly. Integers in Python have an arbitrarily large size. So calculating 2^64 * 2^64 is possible and results in 340282366920938463463374607431768211456. This is very handy indeed from a programmer’s perspective, if perhaps inefficient.

The type of a variable can be determined by the programmer as the program executes. The function type() will return the type of its parameter as a string, and can be printed or tested. So, the code:

z = 1

print (type(z))

z = 1.0

print(type(z))

will result in:

<class 'int'>

<class 'float'>

If one needed to know if z was a float at a particular moment, then:

if type(z)is float:

would do the trick. Type(z) does not return a string, it returns a type. The print() function recognizes that and prints a string, just as it does for True and False. So:

if type(z) == "<class 'float'>":

would be incorrect.

CHAPTER 2

REPETITION

One of the things that makes computers attractive to humans is their ability to do tedious, repetitive tasks accurately and at high speed without getting bored. Humans have to do things repeatedly, and not all of them can be done for us by computers. Brushing our teeth, driving to work, cleaning the carpet—all are repeated actions, and many would be called chores. In programming terms some might be referred to as loops.

Consider a Java while statement; the Python equivalent is very similar, except that instead of a compound statement being enclosed in braces it is indented, as before. The standard is four spaces, but any consistent number will do. All of the actions that follow the while are indented to indicate that they are a part of the activities to be repeated, just as was done in a Python if statement to mark the things that were to be done if the condition was true. This example illustrates one of the Python repetition structures quite accurately: the while statement.

FIGURE 2.1. Essential syntax of the WHILE statement.

Note the lack of parentheses around the expression. Python does not require them.

The WHILE Statement

As in Java and C++, when using this repetition statement the condition is tested at the top or beginning of the loop. If upon that initial test the condition is true, then the body of the loop is executed; otherwise it is not, and the statement following the loop is executed. This means that it is possible that the code in the loop is not executed at all. The condition tested is the same kind of expression that is evaluated in an if statement: one that evaluates to True or False. It could be, and often is, a comparison between two numeric or string values, as it is in the example of Figure 2.1.

When the code in the body of the while statement has been executed, then the condition is tested again. If it is still true, then the body of the loop is executed again, otherwise the loop is exited and the statement following the loop is executed. There is an implication in this description that the body of the loop must change something that is used in the evaluation of the loop condition, otherwise the condition will always be the same and the loop will never terminate. So, here is an example of a loop that is entered and terminates:

a = 0

b = 0

while a < 10:

a = a + 1

print (a)

The condition a<10 is true at the outset because a has the value 0, so the code in the loop is executed. The lone statement in this loop increments a, so that after the first time the loop is executed the value of a is 1. Now the condition is tested and, again, a<10 so the loop executes again. In the final iteration of the loop, the value of a starts out as 9, is incremented, and becomes 10. When the condition is tested it fails, because a is no longer less than 10 (it is equal), and so the loop ends. The statement following the loop is print (a), and the value printed is 10. This loop explicitly modifies one of the variables in the loop condition, and it is easy to see that the loop will end and what the value of a will be at that time.

Here is an example of a loop that is entered and does not terminate:

a = 0

b = 0

while b < 10:

a = a + 1

print (a)

In this case the value of b is less than 10 at the outset so the loop is entered. The body of the loop increments a as before, but does not change b. The loop condition does not depend on a, only on b, so when the loop condition is tested again the value of b is still 0, and the loop executes again. The value of b will always be 0 each time it is tested, so the loop condition will always be true and the loop will never end. The print statement will never be executed.

Here is an example of a loop that is not entered:

a = 100

b = 0

while a < 10:

a = a + 1

print (a)

The condition a<10 is false at the outset because a has the value 100, so the code in the loop is not executed. The statement following the loop is executed next, which is the print statement, and the value printed is 100.

These loops are merely examples that illustrate the three possibilities for a while loop and do not calculate anything useful. The example from the previous chapter can make practical use of a while loop, and it would be useful to look at it again.

Modules and Random Numbers

Most games and simulations depend on an element of unpredictability or chance. We can use a random number to simulate real situations, which are complex enough that they seem random: the distance between cars on the freeway, or the number of customers in a store are examples.

Numbers are random only with respect to each other. Is the number “6” random? That’s not really a good question. Is the sequence 87394 random? Perhaps a test could be devised to answer that. Is the sequence 66666 random? Most would say not, but it has the same probability of being generated at random as does 87394. To create good games and simulations, it is necessary to devise ways to generate a random number using a computer, and to test numbers to see if they are in fact random. Then it would be possible to simulate the flipping of a coin, or the rolling of a die.

Python encompasses the idea of a module, which provides a collection of functions that perform operations within a specified domain. These are Python functions that reside in a file, and it means that the name of the module has to be known as well as the names of the built-in functions within it. As one example, common mathematical functions are located within the module math and can be used by requesting the math module with the statement:

import math

Using a function in the math module involves using the name math followed by a period (“.”) followed by the name of the function. The “.” opens the module so that the names within can be used, because there may be other built-in functions or even variables that have the same name. So, if the statements:

x = math.sqrt(64)

print (x)

are executed, the program will print the number 8, which is the square root of 64. The expression sqrt(64) is a function call, and executes the code needed to calculate the square root of 64. The name sqrt is the name of the function, which is code provided by the Python language. This particular call will always return the value 8, because 8 is always the square root of 64. It is very much like the functions that are studied in grade school mathematics, such as sine and cosine. A module can be thought of as a bag of programs. Each bag contains a set of programs that do a particular class of things, like mathematics or drawing. By specifying the name of the module, access to all of the functions within is granted, and by specifying the specific name of a function, the code that we want is specifically made available.

By the way, the import statement should be at the very beginning of the program.

It is possible to have a function that produces a random number as a value. It is in the module named random, and the function is called random too. For example:

import random

print ( random.random() )

Every time (well almost every time, because it is random, after all) the function is used, it will give a different value, a random value.

This code prints the value 0.07229650795715237. Why? Because random.random() produces a random number between 0.0 and 1.0. This is the most common example of a random number function, and is really very general. It’s the same in Java. Increasing the range is done simply by multiplying by the maximum value desired; random.random()*100 gives a random number between 0 and 100, for instance.

What if the problem is to simulate the roll of a die? The bag of code that is the random module contains other functions related to the generation of random numbers, and one of them is especially suited to this problem. A die roll would be implemented as:

random.randint (1, 6)

The randint function accepts two numbers, called parameters. The first is the lower limit of the range of random integers to be produced, and the second is the upper limit. Specifying 1 as the lower limit and 6 as the upper, as in the example above, means that it will generate numbers between 1 and 6 inclusive, which is what would be expected from rolling a die. The result of rolling two dice would be a number between 2 and 12, found by random.randint(1,6) + random.randint (1,6) (not random.randint(2,12) for reasons of mathematics).

Flipping a coin is a two-level choice, and could be done with random.randint(1,2). More completely:

if (random.randint(1,2) == 1):

print ("Heads")

else:

print ("Tails").

The introduction of a random choice is a little more complicated for the rock-paper-scissors program because the variable holding the player’s choice is a string. There are three possible choices, so to select one at random might look like this:

i = random.randint(1,3)

if i == 1:

choice = "rock"

elif i == 2:

choice = "paper"

else:

choice = "scissors"

Many of the examples that will be developed will involve a game or puzzle of some kind, so the use of random numbers will be a consistent feature of the code shown.

Counting Loops

The while loop is obviously useful, and is in fact the only kind of loop that is required in order to implement any program. However, loops that involve counting a certain number of iterations are pretty common, and adding syntax for this kind of thing would be certain to be valuable for a programmer. Such a construct is the for loop. In some languages a for loop involves a special syntax, but in Python it involves a new type as well (a class of types, really): a tuple. Here is an example of a for loop:

for i in (1,2,3,4,5):

print (i)

This will print the numbers 1 2 3 4 5, each on a separate line. The variable i takes on each of the values in the collection provided in parentheses and the loop executes once for each value of i. The collection (1,2,3,4,5) is called a tuple, and it can contain any Python objects in any order. It’s basically just a set of objects. The following are legal tuples:

(3,6, 9, 12)

(2.1, 3.5, 9.1, 0, 12)

("green", "yellow", "red")

("red", 3, 4.5, 2, "blue", i)

#where i is a variable with a value

The for loop has the loop control variable (in the case above it is i) take on each of the values in the tuple, in left to right order, then executes the connected suite. The loop will therefore execute same the number of times as there are elements in the tuple.

Sometimes it may be necessary to have the loop execute a great many times. If the loop was to execute a million times, it would be more than awkward to require a program to list a million integers in a tuple. Python provides a function to make this more convenient: range(). It returns a tuple that consists of all of the integers between the two parameters it is given, including the lower endpoint. So:

range (1,10) is (1,2,3,4,5,6,7,8,9)

range (-1, 2) is (-1, 0, 1)

range (-1, -3) is not a proper range.

range (1, 1000000) if the set of all integers from 1 to 9999999

Ranges involving strings are not allowed, although tuples having strings in them are certainly allowed. The original example for the loop can now be written:

for i in range(1,6):

print i

and the loop that is to execute a million times could be specified as:

for i in (0, 1000000):

print i

This would print the integers from 0 to 999999. If range() is passed only a single argument, then the range is assumed to start at 0; this means that range (0,10) and range (10) are the same.

FIGURE 2.2. The structure of a FOR statement.

Prime or Non-Prime

Here’s a game that can illustrate the use of a for loop, and some other ideas as well. The computer presents the player with large numbers, one at a time. The player has to guess whether each number is prime or non-prime. A prime number does not have any divisors except 1 and itself. Examples of prime numbers are 3, 5, 11, and 17. The game ends either when a specific number of guesses have been made, or when the player makes a specific number of mistakes.

A key problem to solve in this game is to determine when a number is prime. The computer must be able to determine whether the player is correct, and so for any given number, there must be a way to figure out whether it is prime. Otherwise, the program for this game is not very complicated:

while game is not over:

select a random integer k

print k and ask the player if it is prime

read the player's answer

if player's answer is correct:

print "You are right"

else:

print "You are wrong."

The mysterious portion of this program is the if statement that asks if the player’s answer is correct. This really means that the program must determine whether or not the number K is prime and then see if the player agrees. How can it be determined that a number is prime? A prime number has no divisors, so if one can be found then the number is not prime. The modulo operator % can be used to tell if a division has a remainder: if k % n = 0 then the number n divides evenly into k, and k is not prime.

So to find out whether a number is prime, try dividing it by all numbers smaller than it, and if any of them have a zero remainder then the number is not prime. This is a job for a for loop. Here’s a first draft:

isprime = True

for n in range (1, K):

if k%n == 0:

isprime = False

After the loop has completed, the variable isprime indicates whether K is prime or not. This seems pretty simple, if tedious. It does a lot of divisions. Too many, in fact, because it is not possible for any number larger than K/2 to divide evenly into K. So a slightly better program would be:

isprime = True

# Is the number K prime?

for n in range (1, int(k/2))

# Divide K by all numbers < K/2

if k%n == 0:

# If the remainder is 0 then n

isprime = False

# divides evenly into K: not prime

# If isprime is still true here then the number is prime.

Next, this section of program should be incorporated into a complete program that plays the game. If the game is supposed to allow 10 guesses, then the first step is to repeat the whole thing 10 times:

import random

correct = 0            # The number of correct guesses

for iteration in range(0, 10):  # 10 guesses

Now select a number at random. It should be large enough so that it is hard to see immediately if it is prime, although even numbers are a giveaway:

K = random.randint(10000, 1000000)    # Generate a new number

Next print a message to the user asking for their guess and read it:

print ("Prime or Not: Is the number ",K," prime? (yes or no)") answer = input()      # Read the user's choice

The user types in a string, “yes” or “no,” as their response. The variable isprime that was used in the program that determines whether K is prime is logical, being True or False. It could be made into a string too so that it was the same as what the user typed, and then it could be compared directly against the user’s input:

isprime = "yes"

Now comes the code for determining primality as coded above, except with isprime as a string:

isprime = True

# Is the number K prime?

for n in range (1, int(k/2))

# Divide K by all numbers < K/2

if k%n == 0:

# If the remainder is 0 then n

isprime = "no"

# divides evenly into K: not prime

# If isprime is still true here then the number is prime.

At this point the variable isprime is either “yes” or “no” depending on whether K is actually prime. The user’s guess is also “yes” or “no.” If they are equal then the user guessed correctly.

if isprime==answer:

print ("You are correct!")

correct = correct + 1

else:

print ("You are incorrect.")

Finally, the outer loop is ended and the result is printed. The value of the variable correct is the number of correct guesses the user made, because it was incremented every time a correct answer was detected. The last statement is:

print ("You gave ",correct," right answers out of 10.")

Exiting from a Loop

A clever programmer would notice a pretty serious inefficiency with the prime number program. When it has been determined that the number is not prime, the loop continues to divide more numbers into k until k/2 of them have been tried. If k= 999992 then it is known after the first iteration that the number is not prime; it is even, so it can’t be prime. But the program continues to try nearly another half million numbers anyway. What is needed is a way to tell the program that the loop is over. There is a way to do this.

A loop can be exited using the break statement. It is simply the word break by itself. The correct way to use this in the program above would be:

for n in range (1, int(k/2))

# Divide K by all numbers< K/2

if k%n == 0:

# If the remainder is 0 then n

isprime = "no"

# divides evenly into K: not prime

break

This loop terminates when the number k is known to be not prime. The statement following the loop will be executed next. This can save a lot of computer cycles, but does not make the program more correct—just faster.

A variation on this is the continue statement. This will result in the next iteration of the loop being started without executing any more statements in the current iteration. This avoids doing a lot of work in a loop after it is known it’s not necessary. For example, doing some task for a bunch of names except for people named “Smith” could use a continue statement:

for name in

('Jones','Smith','Peters','Sinatra','Bohr','Conrad'):

print (name);

if name == 'Smith':

continue

# Now do a bunch of stuff . . .

Both break and continue do the same thing in both while and for loops.

Modifying the loop variable will not change the number of iterations the loop will execute. In fact, it has no effect. This loop demonstrates that:

for i in range(0, 10):

print ("Before ",i)

i = i + 1000

print ("After ",i)

It prints:

Before 0

After 1000

Before 1

After 1001

. . .

and so on. It seems that the value of i changes after the assignment for the remainder of the loop and then is set to what it should be for the next iteration. This makes sense if Python is treating the range as a set of elements (it is), and it assigns the next one to i at the beginning of each iteration. Unlike a while loop, there is not a test for continuation. In any case, changing i here does not alter the number of iterations and can’t be used in place of a break.

Else

The idea that the loop can be exited explicitly makes the normal termination of the loop something that should be detectable too. When a while or for loop exits normally by exhausting the iterations or having the expression become False, it is said to have fallen through. When the for loop in the prime number program detects a factor, it executes a break statement, thus exiting the loop. What if it never does that? In that case no factor exists, and the number is prime. The program as it stands has a flag that indicates this, but it could be done with an else clause on the loop.

The else part of a while or for loop is executed only if the loop falls through; that is, when it is not exited through a break. This can be quite useful, especially when the loop is involved in a search, as will be discussed later. In the case of the prime number program, an else could be used when the number is in fact prime, as follows:

for n in range (1, int(k/2))

# Divide K by all numbers< K/2

if k%n == 0:

# If the remainder is 0 then n

isprime = "no"
break

# divides evenly into K: not prime

else:

isprime = "yes"

# Loop not exited: it is prime

An else in a while loop occurs when the condition becomes false. Consider a loop that reads from input until the user types “end” and is searching for the name “Smith”:

inp = input()

while (inp != "Smith"):

s = input()

if s == "end":

break

else:

print ("Smith was found")

# When the program reaches this point it is no

# longer known whether Smith was found.

Of course, the else is not required, and some programmers believe it is even harmful. There are always other ways to accomplish the same thing.

Exceptions and Errors

A correct program depends on the programmer being able to identify all possible circumstances that can occur and knowing how to deal with each of them. Failing to handle one possible situation is an error, and the program will behave unpredictably if that situation occurs in practice. Statements that handle errors appear all through real (commercial) code. In fact, it is common that there are more statements that detect and deal with errors than code that actually computes an answer.

User input is a frequent cause of mistakes in programs. It’s not that the user is the problem; the programmer must anticipate all possible ways that a user can enter data. There is usually one correct way but many erroneous ones, and it is impossible to predict what a user will enter from a keyboard in response to any request. Similarly, the contents of a file may not be what the programmer expects. File formats are standards, but sometimes there are variations, and at other times a user may have entered the data improperly. While the mistake is on the part of the user, it is also a programming mistake if the error is not detected and is allowed to have an impact of the execution of the program.

Most modern languages, Python included, have implemented a way to catch errors and permit the programmer to handle them without having tests before each statement or expression. This facility is called the exception.

The word exception communicates a way to think about how errors will be handled. Some code is legal and calculates a desired value except under certain circumstances, or unless some particular thing happens. The way it works is that the program tries to perform some operation and errors are allowed to occur. If one does, the computer hardware or operating system detects it and tells Python. The program cannot continue in the way that was planned, which is why this is called an exception. The programmer can tell Python what to do if specific errors occur by writing some code that deals with the problem. If the programmer did not do this, then the default is for Python to print an error message that describes the error and then stop executing the program. Error messages can be seen as a failure on the part of the programmer to handle errors correctly.

A simple example is the divide by zero error mentioned previously. If the expression a/b is to be evaluated, the value of b can be checked to make sure it is not zero before the division is done:

if b != 0:

c = a/b

This can be tedious for the programmer if a lot of calculations are being done, and can be error prone. The programmer may forget to test one or two expressions, especially if engaged in modifications or testing. Using exceptions is a matter of allowing the error to happen and letting the system test for the problem. The syntax is as follows:

try:

c = a/b

except:

c = 1000000

The try statement begins a section of code within which certain errors are being handled by the programmer’s code. After that statement, code is indented to show that it is part of the try region. Nearly any code can appear here, but the try statement must be ended before the program ends.

The except statement consists of the keyword except and, optionally, the name of an error. The errors are named by the Python system, and the correct name has to be used, but if no error name is given as in this example, then any error will cause the code in the except statement to be executed. Not specifying a name here is an implicit assumption that either only one kind of error could possibly occur or that no matter what error happens, the same code will be used to deal with it. Specifying an unrecognized name is itself an error. The name can be a variable, but that variable must have been assigned a recognized error name before the error occurs. The code following the except keyword is indented too, to show that it is part of the except statement. This is referred to by programmers as an error handler, and it is executed only if the specified error occurs.

This appears to be even more verbose than testing b, but any number of statements can appear between the try and the except. This section of code is now protected from divide-by-zero errors. If any occur then code following the except statement will be executed; otherwise that code will not execute. If other errors occur then the default action will take place—an error message will be printed.

Testing specifically for the divide-by-zero error can be done by specifying the correct error name in the except statement:

try:

c = a/b

except ZeroDivisionError:

c = 1000000

More than one specific error can be caught in one except statement:

try:

c = a/b

except (ValueError, ZeroDivisionError):

c = 1000000

Clearly (ValueError, ZeroDivisionError) is a tuple, and could be made longer and could be assigned to a variable.

Also, there can be many except statements associated with a single try:

try:

c = a/b

except ValueError:

c = 0

exceptZeroDivisionError:

c = 1000000

And, as was mentioned, a variable can hold the value of the error to be caught:

k = ZeroDivisionError

try:

c = a/b

except k:

c = 1000000

Finally, the exception name can be left out altogether. In that case any exception that occurs will be caught and the exception code will be executed:

try:

c = a/b

except:

c = 0

CHAPTER 3

SEQUENCES: STRINGS, TUPLES, AND LISTS

It was mentioned in Chapter 2 that for loops in Python are different from those found in many other languages in that they use a tuple to define the values that will be assigned to the control variable. Tuples are useful in many situations, and are only one example of a wider range of data types that includes strings, tuples, and lists as objects that consist of multiple parts. They are called sequence types. An integer or a float is a single number, whereas a sequence type consists of a collection of items, each of which is a number or a character. Each member of a sequence is given a number based on its position: the first element in the sequence is given 0, the second is 1, and so on. This is a fundamental data structure in Python and has influenced the syntax of the language.

Strings

A string is a sequence of characters. The word sequence implies that the order of the characters within the string matters, and that is certainly true. Strings most often represent the way that communication between a computer and a human takes place. The order of the characters within a word matters a great deal to a human because some sequences are words and others are not. The string “last” is a word, but “astl” is not. Also, the strings “salt” and “slat” are words and use exactly the same characters as “last” but in a different order.

Because order matters, the representation of a string on a computer will impose an order on the characters within, and so there will be a first character, a second, and so on, and it should be possible to access each character individually. A string will also have a length, which is the number of characters within it. A computer language will provide specific things that can be done to something that is a string: these are called operations, and a type is defined at least partly by what operations can be done to something of that type. Because a string represents text in the human sense, the operations on strings should represent the kinds of things that would be done to text. This would include printing and reading, accessing any character, linking strings into longer strings, searching a string for a particular word, and so on.

String constants are simply characters enclosed in either single or double quotes, similar to those in Java. Assigning a string constant to a variable causes that variable to have the string type and gives it a value. So the statements:

name = "John Doe"

address = '121 Second Street'

cause the variables name and address to be strings with the assigned value. Note that either type of quote can be used, but a string that begins with a double quote must end with one.

A string behaves as if its characters are stored as consecutive characters in memory. The first character in a string is at location or index 0, and can be accessed using square brackets after the string name. Using the definitions above, name[0] is “J” and name[5] = “D.” If an index is specified that is too large, it results in an error because it amounts to an attempt to look past the end of the string.

How many characters are there in the string name? The built-in function len() will return the length of the string. The largest legal index is one less than this value: the first character of a string name has index 0, and the final one has index 7; the length is 8. Thus, any index between 0 and len(name)-1 is legal. The following code prints all of the characters of name and can be thought of as the basic pattern for code that scans through the characters in strings:

for i in range(0, len(name)):

print (name[i], end="")

This may be a little confusing, but remember that the range(0,n) does not include n. This loop runs through values of i from 0 to len(name)-1.

Some languages have a character type, but Python does not. A string of length one is what Python uses instead. A component of a string is therefore another string. The first character of the string name, which is name[0], is “J,” the string containing only one character.

Comparing Strings

Two strings can be compared in the same manner as two integers or real numbers, by using one of the relational operators ==, !=, <, >, <=, or >=. What it means for two strings to be equal is simple and reasonable: if each corresponding character in two strings is the same, then the strings are equal. That is, for strings a and b, if a[0] == b[0], and a[1]==b[1], and so on to the final character n, and a[n] == b[n], then the two strings a and b are equal and a==b. Otherwise a!=b. By the way, this implies that equal strings have the same length.

What about inequalities? Strings in real life are often sorted in alphabetical order. Names in a telephone book, files in a doctor’s office, and books in a store: these tend to appear in a logical order based on the alphabet. This is also true in Python. The string “abc” is less than the string “def,” for example. Why? Because the first letter in “abc” comes before the first letter in “def”; in other words, “abc”[0] < “def”[0]. Yes, characters in string constants can be accessed using their index.

A string s1 is less than string s2 if all characters from 0 through k in the two strings are equal, and s1[k+1]<s2[k+1]. So the following statements are true:

"abcd" < "abce"

"123" < "345"

"ab" < "abc"

In the last example, the space character " " is smaller than (i.e., comes before) the letter “c.” What if the strings are not the same length? The string "ab" < "abc", so if two strings are equal to the end of one of them, then the shorter one is considered to be smaller. These rules are consistent so far with those taught in grade school for alphabetization. Trailing spaces do not matter. Leading spaces can matter, because a space comes before any alphabetic character; that is, " " < "a". Thus "ab" > " z".

As an example that compares strings, consider the following:

a = "J" b = "j"

c = "1"

if b<c:

print ("Lcase < numbers")

else:

print("Lcase > numbers")

if a<c:

print ("Ucase < numbers")

else:

print("Ucase > numbers")

This results in the output:

Lcase > numbers

Ucase > numbers

Problem: Does a City Name, Entered at the Console, Come before or after the Name Denver?

This involves reading a string and comparing it against the constant string “Denver.” Let the input string be read into a variable named city. Then the answer is:

city = input()

if city < "Denver":

print ("The name given comes before Denver in an alphabetic list")

elif city > "Denver":

print ("The name given comes after Denver in an alphabetic list")

else:

print ("The name given was Denver")

If “Chicago” is typed at the console as input, the result is:

Chicago

The name given comes before Denver in an alphabetic list

However, if case is ignored and “chicago” is typed instead, then the result is:

chicago

The name given comes after Denver in an alphabetic list

because, of course, the lowercase “c” comes (as do all lowercase letters) after the upper case “D” at the beginning of “Denver.”

Slicing – Extracting Parts of Strings

To a person a string usually contains words and phrases, which are smaller parts of a string. Identifying individual words is important. To Python this is true also. A Python program consists of statements that contain individual words and character sequences that each have a particular meaning. The words “if,” “while,” and “for” are good examples. Individual characters can be referenced through indexing, but can words or collections of characters be accessed? Yes, if the location (index) of the word is known.

Problem: Identify a “print” Statement in a String

The statement:

print ("Lcase < numbers")

appears in the previous example program. This can be thought of as a string, and assigned to a variable:

statement = 'print ("Lcase < numbers")'

Question: is this a print statement? It is if the first five characters are the word “print.” Each of those characters could be tested individually, but that would be pretty ugly. Python offers a nicer way to do it. A slice is a set of continuous characters within a string. This means their indices are consecutive, and they can be accessed as a sequence by specifying the range of indices within brackets. The previous situation concerning the print statement could be done like this:

if statement[0:5] == "print":

The slice here does not include character 5, but it is 5 characters long including characters 0 through 4 inclusive. A slice from i to j (i.e., x[i:j]) does not include the character at location j. This means that the following lines produce the same result:

fname[0]

fname[0:1]

If the first index is omitted, then the start index is assumed, so the statement:

if statement[0:5] == "print":

is the same as:

if statement[:5] == "print":

If the second index is omitted, then the last legal index is assumed, which is to say the index of the final character. So the assignment:

str = statement[6:]

results in the value of str being “(“Lcase < numbers”).” Both indices can be omitted, which does sound silly, but really just means from the first to the last character, or the entire string.

Editing Strings

Python does not allow the modification of individual parts of a string. That is, things like:

str[3] = "#"

str[2:3] = ".."

are not allowed. So how can strings be modified? For example, consider the string variable:

fname = "image"

If this is supposed to be the name of a JPG image file, then it must end with the suffix “.jpg.”

Problem: Create a JPEG File Name from a Basic String

The string fname can be edited to end with “.jpg” in a few ways, but the easiest one to use is the concatenation operator “+.”

To concatenate means “to link or join together.” If the variables a and b are strings, then a+b is the string consisting of all characters in a followed by all characters in b; the operator “+” in this context means to concatenate, rather than numerical addition. The designers of Python and many other languages that implement this operator think of concatenation as string addition.

To use this to create the image file name, simply concatenate “.jpg” to the string fname:

fname = fname + ".jpg"

The result is that the new value of fname is “image.jpg.”

File suffixes are very often the subject of string manipulations and provide a good example of string editing. For instance, given a file name stored as a string variable fname, is the suffix “.jpg”? Based on the preceding discussion, the question can be answered using a simple if statement:

if fname[len(fname)-4:len(fname)] == '.jpg':

Using a slice it could also take the form:

if fname[len(fname)-4:] == ".jpg"

A valuable thing to know is that negative indices index from the right-hand side of the string; that is, from the end. So fname[-1] is the final character in the string, fname[-2] is the one previous to that, and so on. The last four characters, the suffix, would be captured by using filename[-4:].

Problem: Change the Suffix of a File Name

Some individuals use the suffix “.jpeg” instead of “.jpg.” Some programs allow this; others do not. Some code that would detect and change this suffix would be:

if fname[len(fname)-5:] == ".jpeg":  # identfy jpeg suffix

fname = fname[0:len(fname)-5]  # remove the last 5 char

fname = fname + ".jpg"         # append correct suffix

Problem: Reverse the Order of Characters in a String

There are things about any programming language that could be considered to be ‘idioms.’ These are things that a programmer experienced in the use of that language would consider normal use, but that others might consider odd. This problem exposes a Python idiom. Given what is known so far about Python, the logical approach to string reversal might be as follows:

# city has a legal value at this point

k = len(city)

for i in range(0,len(city)):

city = city + city[k-i-1]

city = city[len(city)//2:]

This reverses the string named city that exists prior to the loop and creates the reversed string. An experienced Python programmer would do this differently. The syntax for taking a slice has a variation that has not been discussed; a third parameter exists. A string slice can be expressed as:

myString[a:b:c]

where a is the starting index, b is the final index+1, and c is the increment. If:

str = "This string has 30 characters."

then str[0:30:2] is “Ti tighs3 hrces,” which is every second character. The increment represents the way the string is sampled, that is, at each increment the character is copied into the result. Most relevant to the current example, the increment can be negative. The idiom for reversing a string is:

print (str[::-1])

As has been explained, the value of str[:] is the whole string. Specifying an increment of 1 implies that the string is scanned from 0 to the end, but in reverse order. This is not intuitive, but it is probably the way that an experienced Python programmer would reverse a string. Any programmer should use the parts of any language that they comprehend very well, and should keep in mind the likely skill set of the people likely to read the code.

Problem: Is a Given File Name That of a Python Program?

A Python program terminates with the suffix ‘.py.’ An obvious solution to this problem is to simply look at the last three characters in the string s to see if they match that suffix:

if s[len(s)-3:len(s)] == '.py':

print ("This is a Python program.")

Perhaps. But is “PROGRAM.PY” a legal Python program? It happens that it is. So is “program.Py” and “program.pY.” What can be done here?

String Methods

A good way to do the test in this case is to convert the suffix to all upper- or all lowercase before doing the comparison. Comparing against “.py” means converted to lowercase, which is done by using a built-in method named lower:

s1 = s[len(s)-3:len(s)]

if s1.lower()== '.py':

print ("This is a Python program.")

The variable s1 is a string that will contain the final three characters of s. The expression s1.lower() creates a copy of s1 in which all characters are lowercase. It’s called a method to distinguish it from a function, but they are very similar things. You should recall that a method is simply a function that belongs to one type or class of objects. In this case lower() belongs to the type (or class) string. There could be another method named lower() that belongs to another class that does a completely different thing. The dot notation indicates that it is a method, and what class it belongs to: the same class of things that the variable belongs to. In addition, the variable itself is really the first parameter; if lower were a function, then it might be called by lower(s1) instead of s1.lower(). In the latter case the “.” is preceded by the first parameter.

Strings all have many methods, and these can be found online or in most Python texts. In the table below the variable s is the target string, the one being operated upon. This means that the method names below will appear following “s.”—for example, s.lower(). Let the value of s be given by s = “hello to you all.” These methods are intended to provide the operations needed to make the string type in Python function as a major communication device from humans to a program.

capitalize() – Returns the target string but with the first letter capitalized.

count(str,beg=0,end=len(s)) – Returns a count of how many times the string str occurs in the target. If values for beg and end are given, then the count is performed using only character indices between beg and end.

endswith(suffix, beg=0, end=len(s)) – Returns True if the target string ends with the given suffix and return False otherwise. If beg and end are given, then do the test on the substring between beg and end.

find(str,beg=0end=len(string)) – If the string str appears with the target string, then return the index at which it occurs; return 1 if it does not occur. If beg and end are provided, then use the substring from beg to end.

isdigit() – Returns True if the target string contains only digits and False otherwise.

islower() – Returns True if the target string has at least one alphabetic character and all alphabetic characters are lowercase. Return False otherwise.

isspace() – Returns True if the target string contains only whitespace characters and returns False otherwise.

isupper() – Returns True if s has at least one alphabetic character and all alphabetic characters are uppercase. Returns False otherwise.

lower() – Converts all uppercase letters in string to lowercase.

replace(old, new [, max]) – Replaces all occurrences of the string old in the target with the string new. If max is specified, replace at most max instances.

split (str=””, num=string.count(str)) – Returns a list of substrings obtained from the target using str as a delimiter. Space is the default for str. Subdivide at most num times if that is specified.

splitlines ( num=string.count(‘\n’)) – Splits the target string at all (or num, if it is specified) NEWLINEs and returns a list of each line with the NEWLINEs removed.

upper() – Converts the lowercase letters in the string to uppercase.

Spanning Multiple Lines

Text as seen in human documents may contain many characters, even multiple lines and paragraphs. A special delimiter, the triple quote, is used when a string constant is to span many lines. This has been mentioned previously in the context of multiline comments. The regular string delimiters will terminate the string at the end of the line. The triple quote consists of either of the two existing delimiters repeated three times. For example, to assign some Python code to the variable code:

code = """list = [1,2,4,7,12,15,21]

for i in list:

print(i, i*2)"""

When code is printed the line endings appear where they were placed in the constant. This example is a particularly good one in that most Python programs require that lines end precisely where the programmer intended.

This program can be executed, too; the following statement will actually execute the code in the string:

exec (code)

For Loops Using Strings

Earlier in this section a for loop was written to print each character in the string. That loop was:

for i in range(0, len(name)):

print (name[i], end="")

Obviously the string could have been printed using:

print(name)

but it was being used as an example of indexing individual components within the string. The characters do not need to be indexed explicitly in Python; the loop variable can be assigned the value of each component:

for i in name:

print (i, end="")

In this case the value of i is the value of the component, not its index. Each component of the string is assigned to i in turn, and there is no need to test for the end of the string or to know its length. This is a better way to access components in a string and, as it happens, can be used with all sequence types. Whether an index is used or the components are pulled out one at a time depends on the problem being solved; sometimes the index is needed, and other times it is not.

The Type Bytes

The type bytes represents a sequence of integers, albeit small ones. A bytes object of length 1 is an 8-bit integer, or a value between 0 and 255. A bytes object of length greater than 1 is a sequence of small integers. To be clear, if s is a string and b is a bytes then:

s[i] is a character

b[i] is a small integer

A string constant (literal) is a sequence of characters enclosed in quotes. A bytes literal is a sequence of characters enclosed in quotes and preceded by the letter ‘b.’ Thus:

'this is a string'

is a string, whereas:

b'this is a string'

has type bytes. Any method that applies to a string also applies to a bytes object, but bytes objects have some new ones. In particular, to convert a bytes object to a string, the decode() method is used, and a character encoding should be given as the parameter. If no parameter is given, then the decoding method is the one currently being used. There are a few possible decoding methods (e.g., “utf-8”). So to convert a bytes object b to a character string s, the following would work:

s = b.decode ("utf-8")

Questions remain: Why is the bytes type needed? What is it used for? Because (and this is a little ahead of what is needed) it implements the buffer interface. Certain file operations require a buffer interface to accomplish their tasks. Anything read from some specific types of file will be of the type bytes, for example, as it has that interface. This will be discussed further in future chapters, but for the moment it simply serves to explain why this sequence type exists at all. Other than the buffer interface, the bytes type is very much like a string, and can be converted back and forth.

Tuples

A tuple is almost identical to a string in basic structure, except that it is composed of arbitrary components instead of characters. The quotes can’t be used to delimit a tuple because a string can be a component, so a tuple is generally enclosed in parentheses. The following are tuples:

tup1 = (2,3,5, 7, 11, 13, 17, 19) # Prime numbers under 20

tup2 = ("Hydrogen","Helium","Lithium","Beryllium", "Boron","Carbon")

tup3 = "hi", "ohio", "salut"

If there is only one element in a tuple, there should be a comma at the end:

tup4 = ("one",)

tup5 = "two",

That’s because it would not be possible otherwise to tell the difference between a tuple and a string enclosed in parentheses. Is (1) a tuple? Or is it simply the number 1?

A tuple can be empty:

tup = ()

Because they are like strings, each element in a tuple has an index, and they begin at 0. Tuples can be indexed and sliced, just like strings. So:

tup1[2:4] is (5, 7)

Concatenation is like that of strings too:

tup4 = tup4 + tup5 # yields tup4 = ('one', 'two')

As is the case with strings, the index 1 gives the last value in the tuple, 2 gives the second last, and so on. So in the example above, tup2[-1] is “Carbon.” Also, like strings, the tuple type is immutable; this means that elements in the tuple can’t be altered. Thus, statements such as:

tup1[2] = 6

tup3[1:] "bonjour"

are not allowed and will generate an error.

Tuples are an intermediate form between strings, which have just been discussed, and lists, which will be discussed next. They are simpler to implement than list (are lightweight) and are more general than strings.

Tuples underlie other aspects of Python.

Tuples in For Loops

Sequences can be used in a for loop to control the iteration and assign to the loop control variable. Tuples are interesting in this context because they can consist of strings, integers, or floats. The loop:

for i in ("Hydrogen","Helium","Lithium","Beryllium",

"Boron","Carbon"):

will iterate six times, and the variable i will take on the values in the tuple in the order specified. The variable i is a string in this case. In cases where the types in the tuple are mixed, things are more complicated.

Problem: Print the Number of Neutrons in an Atomic Nucleus

Consider the tuple:

atoms=("Hydrogen",1,"Helium",2,"Lithium",3,

"Beryllium",4,"Boron",5,"Carbon",6)

and the loop

for i in atoms:

print (i)

This prints:

Hydrogen

1

Helium

2

Lithium

3

Beryllium

4

Boron

5

Carbon

6

The number following the name of the element is the atomic number of that element, the number of protons in the nucleus. In this case the type of the variable i alternates between string and integer. For elements with a low atomic number (less than 21), a good guess for the number of neutrons in the nucleus is twice the number of protons. The complexity is that some of the components are strings and some are integers. The program should only do the calculation when it is in an iteration having an integer value for the loop variable, because a string can’t be multiplied by two.

A built-in function that can be of assistance is isinstance. It takes a variable and a type name and returns True if the variable is of that type and False otherwise. Using this function, here is a first stab at a program that makes the neutron guess:

atoms=("Hydrogen",1,"Helium",2,"Lithium",3,

"Beryllium",4,"Boron",5,"Carbon",6)

for i in atoms:

if isinstance(i, int): # Is i an integer?

j = i*2

print ("has ", i, "protons and ", j, " neutrons.")

else:

print ("Element ", i)

In other words, in iterations where i is an integer as determined by isinstance, then i can legally be multiplied by 2 and the guess about number of neutrons can be printed.

Another way to solve the same problem would be to index the elements of the tuple. Elements 0,2,4, and so on (even indices) refer to element names, while the others refer to atomic numbers. This code would look as follows:

atoms=("Hydrogen",1,"Helium",2,"Lithium",3,

"Beryllium",4,"Boron",5,"Carbon",6)

for i in range(0,len(atoms)):

if i%2 == 1:

j = atoms[i]*2

print ("has ", atoms[i], "protons and ",

j, " neutrons.")

else:

print ("Element ", atoms[i])

Note that in this case the loop variable is always integer, and is not an element of the tuple but is an index at which to find an element. That’s why the expression atoms[i] is used inside the loop instead of simply i as before.

Membership

Tuples are not sets in the mathematical sense, because an element can belong to a tuple more than once, and there is an order to the elements. However, some set operations could be implemented using tuples by looking at individual elements: set union and intersection, for example. The intersection of two sets A and B is the set of elements that are members of A and also members of B. The membership operator for tuples is the key word in:

if 1 in tuple1: # 1 is an entry in tuple1

The intersection of A and B, where A and B are tuples, could be found using the following code:

for i in A:

if i in B:

C = C + i

The tuple C will be the intersection of A and B. It works by taking each known element of A and testing to see if it is a member of B; if so, it is added to C.

Problem: What Even Numbers Less than or Equal to 100 Are Also Perfect Squares?

This could be expressed as a set intersection problem. The set of even numbers less than 100 could be enumerated (this is not actual code):

A = (2,4,6,8,10 . . . and so on

or could be generated within a loop:

A = ()

# Start with an empty tuple

for i in range(0,51):

# for appropriate integers

A = A + (i*2,)

# add the next even number to the tuple

# Can't simply use A+i because i is integer, not a tuple.

Similarly, the perfect squares could be enumerated:

B = (4,9,16,25,36,49,64,81,100)

Or, again, created in a loop:

B = ()

for i in range(0,11):

B = B + ((i*i),)

Now the set A can be examined, element by element, to see which members also belong to B:

C = ()

for i in A:

if i in B:

C = C + (i,)

The result is: (0, 4, 16, 36, 64, 100).

Two important things are learned from this. First, when constructing a new tuple from components, one can begin with an empty tuple. Second, individual components can be added to a tuple using the concatenation operator “+,” but the element should be made into a tuple with one component before doing the concatenation.

Delete

A tuple is immutable, meaning that it cannot be altered. Individual elements can be indexed but not changed or deleted. What can be done to create a new tuple that has new elements; in particular, deleting an element means creating a new tuple that has all of the other elements except the one being deleted.

Problem: Delete the Element Lithium from the Tuple atoms, along with Its Atomic Number

Going back to the tuple atoms, deleting one of the components—in particular, Lithium—begins with determining which component Lithium is; that is, what is its index? So start at the first element of the tuple and look for the string “Lithium,” stopping when it is found.

for i in range(0, len(atoms)):

if atoms[i] == "Lithium": # Found it at location i

break;

else:

i = -1                    # not found

Knowing the index of the element to be deleted, it is also known that all elements before that one belong to the new tuple and all elements after it do too. The elements before element i can be written as atoms[0:i]. Each element consists of a string and an integer, and assuming that both are to be deleted means that the elements following element i are atoms[i+2:]. In general to delete one element the second half would be atoms[i+1:]. Finishing the code that deletes “Lithium”:

if i>=0:

atoms = atoms[0:i] + atoms[i+2:]

So the tuple atoms has not been altered so much as it has been replaced completely with a new tuple that has no Lithium component.

Update

Again, because a tuple is immutable individual elements cannot be changed. A new tuple can created that has new elements; in particular, updating an element means creating a new tuple that has all of the other elements except the one being updated, and that includes the new value in the correct position.

Problem: Change the Entry for Lithium to an Entry for Oxygen

An update is usually a deletion followed by the insertion or addition of a new component. A deletion was done in the previous section, so what remains is to add a new component where the old one was deleted. Inserting the element Oxygen in place of Lithium would begin in the same way as the simple deletion already implemented:

for i in range(0, len(atoms)):

if atoms[i] == "Lithium": # Found it at location i

break;

else:

i = -1                    # not found

Next, a new tuple for Oxygen is created:

newtuple = ("Oxygen", 8)

Finally, this new tuple is placed at location i, while Lithium is removed:

if i>=0:

atoms = atoms[0:i] + newtuple + atoms[i+2:]

However, an update may not always involve a deletion. If Lithium is not a component of the tuple atoms, then perhaps Oxygen should be added to atoms anyway. Where? How about at the end?

else: # If i is -1 then the new tuple goes at the end

atoms = atoms + newtuple

Tuple Assignment

One of the unique aspects of Python is so-called tuple assignment. When a tuple is assigned to a variable, the components are converted into an internal form that is the one tuples always use. This is called tuple packing, and it has already been encountered:

atoms=("Hydrogen",1,"Helium",2,"Lithium",3,

"Beryllium",4,"Boron",5,"Carbon",6)

What is really interesting is that tuple unpacking can also be used. Consider the tuple:

srec = ('Parker', 'Jim', 1980, 'Math 550', 'C+', 'Cpsc 302', 'A+')

which is a tuple packing of a student record. It can be unpacked into individual variables in the following way:

(fname, lname, year, cmin, gmin, cmax, gmax) = srec

Which is the same as:

fname = srec[0]

lname = srec[1]

year = srec[2]

cmin = srec[4]

gmin = srec[5]

cmax = srec[6]

gmax = srec[7]

Of course, the implication is that N variables can be assigned the value of N expressions or variables “simultaneously” if both are written as tuples. Examples would be:

(a, b, c, d, e) = (1,2,3,4,5)

(f, g, h, i, j) = (a, b, c, d, e)

The expression

(f, g, h, i, j) = 2 ** (a,b,c,d,e)

is invalid because the left side of “**” is not a tuple, and Python won’t convert 2 into a tuple. Also:

(f, g, h, i, j) = (2,2,2,2,2) ** (a,b,c,d,e)

is also invalid because “**” is not defined on tuples, nor are other arithmetic operations. As with strings “+” means concatenation, though, so (1,2,3) + (4,5,6) yields (1,2,3,4,5,6).

Exchanging values between two variables is a common thing to do. It’s an essential part of a sorting program, for example. The exchange in many languages requires three statements: a temporary copy of one of the variables has to be made during the swap:

temp = a

a = b

b = temp

Because of the way that tuples are implemented, this can be performed in one tuple assignment:

(a,b) = (b,a)

This is a little obscure—not to an experienced Python programmer, but certainly to a beginner. A Java programmer could see what was meant, but initially the rationale would not be obvious. This statement deserves a comment such as “perform an exchange of values using a tuple assignment.”

Built-In Functions for Tuples

As examples for the table below, use the following:

T1 = (1,2,3,4,5)

T2 = (-1,2,4,5,7)

len(T1) – Gives the number of components that are members of T1.

max(T1) – Returns the largest element that is a component of T1.

min(T1) – Returns the smallest element that is a component of T1.

In addition, tuples can be compared using the same operators as for integers and strings. Comparison is done on an element-by-element basis, just as it is with strings. In the example above T1>T2 because at the first location where the two tuples differ (the initial component), the element in T1 is greater than the corresponding element in T2. It is necessary for the corresponding elements of the tuple to be comparable; that is, they need to be of the same type. So if the tuples t1 and t2 are defined as:

t1 = (1, 2, 3, "4", "5")

t2 = (-1,2,4,5,7)

then the expression t1>t2 is not allowed. A string can’t be compared against an integer, and element 3 of t1 is a string, whereas element 3 of t2 is an int.

Lists

One way to think of a Python list is that it is a tuple in which the components can be modified. They have many properties of an array of the sort one might find in Java or C, in that they can be used as a place to store things and have random access to them; any element can be read or written. They are often used as one might use an array, but have a greater natural functionality.

Initially a list looks like a tuple, but uses square brackets.

list1=[2,3,5,7, 11, 13, 17, 19] # Prime numbers under 20

list2=["Hydrogen","Helium","Lithium","Beryllium","Boron", "Carbon"]

list3=["hi", "ohio", "salut"]

A list can be empty:

list4 = []

and because they are like tuples and strings, each element in a list has an index, and they begin (as usual) at 0. Lists can be indexed and sliced, as before:

list1[2:4] is [5, 7]

Concatenation is like that of strings too:

list6 = list1 + [23, 31]

yields [2, 3, 5, 7, 11, 13, 17, 19, 23, 31]

Negative values index from the end of the string. However, unlike strings and tuples, individual elements can be modified. So:

list1[2] = 6

results in list1 being [2, 3, 6, 7, 11, 13, 17, 19]. Also:

list3[1:] = "bonjour"

results in list3 taking the value—oops, it becomes:

[‘hi’, ‘b’, ‘o’, ‘n’, ‘j’, ‘o’, ‘u’, ‘r’].

That’s because a string is a sequence too, and this string consists of seven components. Each component of the string becomes a component of the list. If the string “bonjour” is supposed to become a single component of the list, then it needs to be done this way:

list3[1:] = ["bonjour"]

where the component is clearly defined as a list. The other components of list3 are sequences, and now so is the new one. However, integers are not sequences, and the assignment:

list1[2] = [6,8,9]

results in the value of list2 being:

[2, 3, [6, 8, 9], 7, 11, 13, 17, 19]

There is a list within this list; that is, the third component of list1 is not an integer, but is a list of integers. That’s legitimate, and works for tuples as well, but may not be what is intended.

Problem: Compute the Average (Mean) of a List of Numbers

The mean is the sum of all numbers in a collection divided by the number of numbers. If a set of numbers already exists as a list, calculating the mean might involve a loop that sums them followed by a division. For example, assuming that list1 = [2, 3, 5, 7, 11, 13, 17, 19]:

mean = 0.0

for i in list1:

mean = mean + i

mean = mean/len(list1)

It can be seen that a list can be used in a loop to define the values that the loop variable i will take on, a similar situation to that of a tuple. A second way to do the same thing would be:

mean = 0.0

for i in range(0,len(list1)):

mean = mean + list1[i]

mean = mean/len(list1)

In this case the loop variable i is an index into the list and not a list element, but the result is the same. Python lists are more powerful than this, and making use of the extensive power of the list simplifies the calculation greatly:

mean = sum(list1) / len(list1)

The built-in function sum will calculate and return the sum of all of the elements in the list. That was the purpose of the loop, so the loop is not needed at all. The functions that work for tuples also work for lists (min, max, len), but some of the power of lists is in the methods a list provides.

Editing Lists

Editing a list means to change the values within it, usually to reflect a new situation to be handled by the program. The most obvious way to edit a list is to simply assign a new value to one of the components. For example:

list2 = ["Hydrogen","Helium","Lithium","Beryllium", "Boron","Carbon"]

list2[0] = "Nitrogen"

print (list2)

results in the following output:

[‘Nitrogen’, ‘Helium’, ‘Lithium’, ‘Beryllium’, ‘Boron’, ‘Carbon’]

This substitution of a component is not possible with strings or tuples. It is possible to replace a single component with another list:

list2 = ["Hydrogen","Helium","Lithium","Beryllium", "Boron","Carbon"]

list2[0] = ["Hydrogen", "Nitrogen"]

results in:

list2 = [['Hydrogen','Nitrogen'],'Helium','Lithium',

'Beryllium','Boron','Carbon']

Insert

To place new components within a list, the insert method is provided. This method places a component at a specified index; that is, the index of the new element will be the one given. To place “Nitrogen” at the beginning of list2, which is index 0:

list2.insert(0, "Nitrogen")

The first value given to insert, 0 in this case, is the index at which to place the component, and the second value is the thing to be inserted. Inserting “Nitrogen” at the end of the list would be accomplished by:

list2.insert(len(list2), "Nitrogen")

However, consider this:

list2.insert(-1, "Nitrogen")

Will this insert “Nitrogen” at the end? No. At the beginning of the statement, the value of list2[-1] is “Carbon.” This is the value at index 5. Therefore, the insertion of “Nitrogen” will be at index 5, resulting in:

[‘Hydrogen’, ‘Helium’, ‘Lithium’, ‘Beryllium’, ‘Boron’, ‘Nitrogen’, ‘Carbon’]

Append

A way to add something to the end of a list is to use the append method:

list2.append("Nitrogen")

results in:

[‘Hydrogen’, ‘Helium’, ‘Lithium’, ‘Beryllium’, ‘Boron’, ‘Carbon’, ‘Nitrogen’]

Remember, the “+” operation will only concatenate a list to a list, so the equivalent expression involving “+” would be:

list2 = list2 + ["Nitrogen"]

Extend

The extend method does pretty much the same things as the “+” operator. With the definitions:

a = [1,2,3,4,5]

b = [6,7,8,9,10]

print (a+b)

a.extend(b)

print(a)

the output is:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

However, if append had been used instead of extend:

a = [1,2,3,4,5]

b = [6,7,8,9,10]

print (a+b)

a.append(b)

print(a)

the result would have been:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

[1, 2, 3, 4, 5, [6, 7, 8, 9, 10]]

Remove

The remove method does what is expected: it removes an element from the list. But unlike insert, for example, it does not do it using an index; the value to be removed is specified. So:

list1 = ["Hydrogen","Helium","Lithium",

"Beryllium","Boron","Carbon"]

list1.remove("Helium")

results in the list1 being [‘Hydrogen’, ‘Lithium’, ‘Beryllium’, ‘Boron’, ‘Carbon’]. Unfortunately, if the component being deleted is not a member of the list, then an error occurs. There are ways to deal with that, or a test can be made for trying to delete an item:

if "Nitrogen" in list1:

list1.remove("Nitrogen")

If there is more than a single instance of the item being removed, then only the first one will be removed.

Index

When discussing tuples it was learned that the index method looked through the tuple and found the index at which a specified item occurred. The index method for lists works in the same way. So:

list1 = ["Hydrogen","Helium","Lithium",

"Beryllium","Boron","Carbon"]

print (list1.index("Boron"))

prints “4,” because the string “Boron” appears at index 4 in this list (starting from 0, of course). If there is more than one occurrence of “Boron” in the list, then the index of the first one (i.e., the smallest index) is returned. If the value is not found in the string, then an error occurs. Again, it might be appropriate to check:

if "Boron" in list1:

print (list1.index("Boron"))

Sort

This method places the components of a list into ascending order. Using the list1 variable that has been used so often:

list1 = ["Hydrogen","Helium","Lithium",

"Beryllium","Boron","Carbon"]

list1.sort()

print(list1)

the result is:

[‘Beryllium’, ‘Boron’, ‘Carbon’, ‘Helium’, ‘Hydrogen’, ‘Lithium’]

which is in alphabetical order. The method will sort integers and floating-point numbers as well. Strings and numbers cannot be mixed, though, because they can’t be compared. So:

list2 = ["Hydrogen",1,"Helium",2,"Lithium",3,

"Beryllium",4,"Boron",5]

list2.sort()

results in an error that will be something like

list2.sort()

TypeError: unorderable types: int() < str()

The meaning of this error should be clear. Things of type int (integer) and things of type str (string) can’t be compared against each other and so can’t be placed in a sensible order if mixed. For sort to work properly, all of the elements of the list must be of the same type. It is always possible to convert one type of thing into another, and in Python converting an integer to a string is accomplished with the str() function; string to integer is converted using int(). So str(3) would result in “3,” and int(“12”) is 12. An error will occur if it is not possible, so int(12.2) will fail.

If each element of a list is itself a list, it can still be sorted. Consider the list:

z = [["Hydrogen",3], ["Hydrogen",2], ["Lithium",3],

["Beryllium",4], ["Boron",5]]

When sorted this becomes:

[['Beryllium',4],['Boron',5],['Hydrogen',2],['Hydrogen',3],

['Lithium',3]]

Each component of this list is compatible with the others, consisting of a string and an integer. Thus, they can be compared against each other. Notice that there are two entries for hydrogen: one with a number 2 and one with a number 3. The sort method arranges them correctly. A list is sorted by individual elements in sequence order, so the first thing tested would be the string. If those are the same, then the next element is checked. That’s an integer, so the component with the smallest integer component will come first.

Reverse

In any sequence the order of the components within it is important. Reversing that order is a logical operation to provide, but it may not be used very often. One instance where it can be important is after a sort. The sort method always places components into ascending order. If they are supposed to be in descending order, then the reverse method becomes valuable. As an example consider sorting the list q:

q = [5, 6, 1, 5, 4, 9, 9, 1, 6, 3]

q.sort()

The value of q at this point is

[1, 1, 3, 4, 5, 5, 6, 6, 9, 9]

To place this list is descending order the reverse method is used:

q.reverse()

and the result is

[9, 9, 6, 6, 5, 5, 4, 3, 1, 1]

Count

This method is used to determine how many times a potential component of a list actually occurs. It does not return the number of elements in the list—that job is done by the len function. Using the list q as an example:

q = [5, 6, 1, 5, 4, 9, 9, 1, 6, 3]

print (1,q.count(1), 2, q.count(2), 3,

q.count(3), 99, q.count(99))

will result in the output:

12                  20                  31              990

where the spacing is enhanced for emphasis. This says that there are 2 instances of the number 1 (1,2) in the list, zero instances of 2 (2,0), one instance of the number 3 (3,1) and none of 99 (99,0).

List Comprehension

When creating a list of items, two mechanisms have been discussed. The first is to use constants, as in the list q in the previous section. The second appends items to a list, and this could be done within a loop. Making a list of perfect squares could be done like this:

t = []

for i in range(0,10):

t = t + [i*i]

which creates the list [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]. This kind of thing is common enough that a special syntax has been created for it in Python—the list comprehension.

The basic idea is simple enough, although some specific cases are complicated. In the previous situation involving perfect squares, the elements in the list are some function of the index. When that is true the loop, index, and function can be given within the square brackets as a definition of the list. So, the list t could be defined as:

tt = [i**2 for i in range(10)]

The for loop is within the square brackets, indicating that the purpose is to define components of the list. The variable i here is the loop variable, and i**2 is the function that creates the elements from the index. This is a simple example of a list comprehension.

Creating random integer values? No problem:

tt = [random.randint(0,100) for i in range(10)]

The first six elements in all uppercase?

list1 = ["Hydrogen","Helium","Lithium","Beryllium",

"Boron","Carbon"]

ss = [i.upper() for i in list1]

This is a very effective way to create lists, but it does depend on having a known connection between the index and the element.

Lists and Tuples

A tuple can be converted into a list. Lists have a greater functionality than tuples; that is, they provide more operations and a greater ability to represent data. On the other hand, they are more complicated and require more computer resources. If something can be represented as a tuple, then it is likely best to do so. A tuple is designed to be a collection of elements that as a whole represent some more complicated object, but that individually are perhaps of different types. This is rather like a C struct or Pascal record. A list is more often used to hold a set of elements that all have the same type, more like an array. This is a good way to think of the two types when deciding what to use to solve a specific problem.

Python provides tools for conversion. The built-in function list takes a tuple and converts it into a list; the function tuple does the reverse, taking a list and turning it into a tuple. For example, converting list1 into a tuple:

tuple1 = tuple(list1)

print(tuple1)

yields

(‘Hydrogen’, ‘Helium’, ‘Lithium’, ‘Beryllium’, ‘Boron’, ‘Carbon’)

This is seen to be a tuple because of the ‘(’ and ‘)’ delimiters. The reverse:

v = list(tuple1)

print(v)

prints the text line:

[‘Hydrogen’, ‘Helium’, ‘Lithium’, ‘Beryllium’, ‘Boron’, ‘Carbon’]

and the square brackets indicate this is a list.

Exceptions

Exceptions are the usual way to check for errors of indexing and membership in lists. The error is allowed to occur, but an exception is tested and handled in the case where, for example, an item being deleted is not in the list.

Problem: Delete a Specified Element from a List

Given the same list, read an element from the keyboard and delete that element from the list. The basic code is the same, but now the string is entered and could be anything at all. It’s easier to test a program when it can be made to fail on purpose. The name is entered using the input function and is used as the parameter to remove. Now it is possible to test all of the code in this program without changing it. First, here is the program:

list1 = ["Hydrogen","Helium","Lithium","Beryllium",

"Boron","Carbon"]

s = input("Enter:")

try:

list1.remove(s)

except:

print ('Can't find ', s)

print (list1)

Properly testing a program means executing all of the statements that comprise it and ensuring that the answer given is correct. So in this case, first delete an element that is a part of the list. Try Lithium. Here is the output:

Enter: Lithium

[‘Hydrogen’, ‘Helium’, ‘Beryllium’, ‘Boron’, ‘Carbon’]

This is correct. These are the statements that were executed in this instance:

list1 = ["Hydrogen","Helium","Lithium","Beryllium",

"Boron","Carbon"]

s = input("Enter:")

try:

list1.remove(s) # This was successful

print (list1)

Now try to delete Oxygen. Output is:

Enter: Oxygen

Can’t find Oxygen

[‘Hydrogen’, ‘Helium’, ‘Lithium’, ‘Beryllium’, ‘Boron’, ‘Carbon’]

This is correct. These statements were executed:

list1 = ["Hydrogen","Helium","Lithium","Beryllium",

"Boron","Carbon"]

s = input("Enter:")

try:

list1.remove(s) # this was not successful

except:

print ('Can't find ', s)

print (list1)

All of the code in the program has been executed and the results checked for both major situations. For any major piece of software, this kind of testing is exhausting, but it is really the only way to minimize the errors that remain in the final program.

Set Types

Something of type set is an unordered collection of objects. An element can only be a member of a given set once, so in that sense it is much like a mathematical set. In fact, that’s the point. Because a set is unordered, operations such as indexing and slicing are not provided. It does support membership (is), size (len()), and looping on membership (for i in set).

Anyone (probably an older person) who knows the Pascal language has some familiarity with the set type in Python.

Mathematical sets have certain specific, well-defined operations, and those are available on a Python set also.

Subset

set1 < set2 means set1 is a true subset of s2.

Intersection

set1 & set2 creates a new set containing members in common with both.

Union

set1 | set2 creates a new set with all elements of both.

Difference

set1-set2 creates a new set with members that are not in both.

Equality

set1==set2 is true if both sets contain only the same elements.

Creating a new object of type set is a matter of specifying either that it is a set or what the elements are. So, one way is to use the {} syntax:

set1 = {1,3,5,7,9}

or to use the constructor:

set2 = set(range(1, 10))

which gives the set {1, 2, 3, 4, 5, 6, 7, 8, 9}. So:

set1<set2 is True

set1 & set2 is {9, 1, 3, 5, 7} Note: order does not matter to a set.

set1 | set2 is {1, 2, 3, 4, 5, 6, 7, 8, 9}

set2 – set1 is {8, 2, 4, 6}

A new element can be added to a set using add():

set1.add(11)

and removed using remove():

set1.remove(11)

or discard():

set1.discard(11)

If the element being removed is not in the set, then an error will occur (KeyError) when remove() is called, but not with discard(). This should be tested first or be placed in an except statement.

All of the examples so far involve integers belonging to a set, but other types can belong as well: floating-point numbers, strings, and even tuples (not lists). For example, the following are legal sets:

{"a", "e", "i", "o", "u"}

{"cyan", "yellow", "magenta"}

{(2,4), (3,9), (4,16), (5,25), (6,36), (7,49)}

Example: Craps

Craps is a dice game, for those unfamiliar with it, and commonly involves betting on the outcome. The player (shooter) rolls two dice. If, on the first roll (pass), a total of 7 or 11 is obtained, then the shooter wins. On the other hand, an initial roll of 2, 3, or 12 loses immediately. Any other roll is called the point. In that case the shooter continues to roll the dice. If a 7 is obtained then the shooter loses, and if the point number is rolled then the shooter wins. The shooter continues to roll until one or the other occurs. One way to implement this game in Python is to use sets.

Elements of the sets will be values on each die, which is to say one roll. There are two dice so a total of 36 combinations exist. A single roll is a tuple, such as (1,1) or (3,4). There are only 12 distinct sums of two dice, and multiple ways to achieve them. A sequence named roll will be created that contains a set for each possible value, and that set contains all of the ways that the value can be obtained. For instance, there are two ways to roll a 3, so:

roll[3] = {(1,2), (2,1)}

Initially a set is created for each possible roll of a pair of dice and then is initialized as described:

from random import *

roll = list(range(0,13))

# Create the empty list

for i in range(1,13):

# and fill with empty sets.

roll[i] = set()

for i in range (1,7):

# Now for each possible roll

for j in range (1,7):

# of two dice, add that roll

k = i+j

# to the element of roll for

roll[k].add( (i,j) )

# that value (sum of the dice)

Now roll[i] contains all of the ways to roll a value of i. In particular, roll[7] contains all ways to roll a 7 and roll[11] contains all ways to roll an 11. Thus, all of the rolls that will win on the first pass can be placed in a single set, the union of roll[7] and roll[11]:

winner = roll[7] | roll[11]

Similarly the rolls that will lose for the shooter on the first pass are:

loser = roll[2] | roll[3] | roll[12]

If any other roll is thrown, then that becomes the point. Rolling a die amounts to getting a random number between 1 and 6 inclusive, or:

die1 = randrange(1,7)

die2 = randrange(1,7)

Remember that randrange() produces a number less than the second parameter. Given this roll, the point is the set roll[die1+die2]. Continuing the program from the die rolls:

val = (die1,die2)               # A tuple, the current roll

print ("Shooter rolls ", val)

if val in winner:               # Is this tuple a winner?

print ("The shooter wins!")

elif val in loser:              # Is it a loser?

print ("The shooter loses")

else:

point = roll[die1+die2]      # Define the point set

print (die1+die2, " is your point.")

Now the dice are rolled repeatedly. If the roll is in the point set, then the shooter wins. If the roll is a 7 (in the set roll[7]) then the player loses. Otherwise the shooter rolls again.

while True:     # Repeat until a win or loss happens

die1 = randrange(1,7)  # Roll the dice

die2 = randrange(1,7)

val = (die1, die2)   # val is a tuple

print ("Rolls ", val)

if val in roll[7]:   # Any 7 roll loses

print ("The shooter loses!")

break

if val in point:   # Rolling the 'point' wins.

print ("The shooter makes the point. A winner!")

break

In a real craps game this entire process is repeated, and bets are placed.

CHAPTER 4

FUNCTIONS

Unlike in the cases of if statements or for statements, a function definition in Python does not involve the word ‘function.’ As an example of a simple definition, imagine a program that needs a function to print twenty “#” characters on a line. It could be defined as:

def pound20 ():

for i in range(0,20):

print ("#", end="")

The word def is known to Python and always begins the definition of a function. This is followed by the name of the function, in this case pound20 because the function prints 20 pound characters (“#”). Then comes the list of parameters, which can be thought of as a tuple of variable names. In this case the tuple is empty, meaning that nothing is passed to the function. Finally comes the “:” character that defines a new suite that comprises the code belonging to the function. From here on the code is indented one more level, and when the indentation reverts to the original level, the function definition is complete.

Calling this function is a matter of using its name as a statement or in an expression, being careful to always include the tuple of parameters. Even when the tuple is empty it helps distinguish a function from a variable. A call to this function would be:

pound20 ()

and the result would be that 20 “#” characters would be printed on one line of the output console.

FIGURE 4.1. The syntax of a function definition.

A function can be passed one or more values that will determine the result of the function. A function cosine, for example, would be passed an angle, and that angle would be used to compute the cosine. Each call to cosine passing a different value can yield a different result. In the case of the function that prints pound characters, it might be useful to pass it the number of pound characters to print. It should not be called pound20 anymore, because it does not always print 20 characters. It will be called poundn this time:

def poundn (ncharacters):

for i in range(0,ncharacters):

print ("#", end="")

The variable ncharacters that is given in parentheses after the function name is called a parameter or an argument, and indicates the name by which the function will refer to the value passed to it. This name is known only inside of the function, and while it can be modified within the function, this modification will not have any bearing on anything outside. The call to poundn must now include a value to be passed to the function:

poundn (3)

When this call is performed the code within poundn begins executing, and the value of ncharacters is 3, the value that was passed. It prints 3 characters and returns. A subsequent call to poundn could be passed a different number, perhaps 8, and then ncharacters would take on the value 8 and the function would print 8 characters. It will print as many characters as requested through the parameter.

NOTE

A def statement is not a declaration. Such things are foreign to Python. A def statement executes, and it effectively “creates” a new function each time it is executed.

Function Execution

All functions return a value, and as such can be treated within expressions as if they were variables. They cannot be assigned to, but otherwise have the same utility. So, assuming the existence of a cosine function, it could be used in an expression in the usual ways. For example:

x = cosine(x)*r

while cosine(x) < 0.5:

print (cosine(x)*cosine(x))

In these cases the value returned by the function is used by the code to calculate a further value or to create output. The expression “cosine(x)” resolves to a value of some Python type. The most common purpose of a function is to calculate a value, which is then returned to the calling part of the program and will possibly be used in a further calculation.

The return statement assigns a value and a type to the object returned by the function. It also stops executing the function and resumes execution at the location where the function was called. A simple example would be to return a single value, such as an integer or floating-point number:

return 0

returns the value 0 from a function. The return value could be an expression:

return x*x + y*y

A function has only one return value, but it can be of any type, so it could be a list or tuple that contains multiple components:

return (2,3,5,7,11)

return ["fluorine","chlorine","bromine","iodine"]

Expressions can include function calls, so a return value can be defined in this way as well; for example

return cosine(x)

One of the simplest functions that can be used as an example is one that calculates the square of its parameter. It nonetheless illustrates some interesting things:

def square (x):

return x*x

The print statement:

print (square(12))

will print:

144

Interestingly, the statement:

print(square(12.0))

results in:

144.0

The same function returns an integer in one case and a float in the other. Why? Because the function returns the result of an expression involving its parameter, which in one case was integer and in the other was real. This implies that a function has no fixed type, and can return any type at all. Indeed, the same function can have return statements that return an integer, a float, a string, and a list independent of type of the parameter passed:

def test (x): # Return one of four types depending on x

if x<1:

return 1

if x<2:

return 2.0

if x<3:

return "3"

return [1,2,3,4]

print (test(0))

print (test(1))

print (test(2))

print (test(3))

The output:

1

2.0

3

[1, 2, 3, 4]

Problem: Write a Function to Calculate the Square Root of Its Parameter

Two thousand years ago the Babylonians had a way to calculate the square root of a number. They understood the definition of a square root: that if y*y = x then y is the square root of x. They figured out that if y was an overestimate to the true value of the square root of x, then x/y would be an underestimate. In that case, a better guess would be to average those two values: the next guess would be y1 = (y + x/y)/2. The guess after that would be y2 = (y1+x/y1)/2, and so on. At any point in the calculation the error (difference between the correct answer and the estimate) can be found by squaring the guess yi and subtracting x from it, knowing that yi*yi is supposed to equal x.

The function will therefore start by guessing what the square root might be. It cannot be 0 because then x/y would be undefined. x is a good guess. Then construct a loop based on the expression y2 = (y1+x/y1)/2, or more generally yi+1 = (yi+x/yi)/2 for iteration i. At first, run this loop a fixed number of times, perhaps 20. Here is the function that results:

def root (x):

# Compute the square root of x

y = x

# First guess: too big, probably

for i in range(1, 20):

# Iterate20 times

y = (y + x/y)/2.0

# Average the prior guess and x/y

return y

# Return the last guess

This correctly computes the square root of 2 to 15 decimal places. This is probably more than is necessary, meaning that the loop is executing more times than it needs to. In fact, changing the 20 iterations to only 6 still gives 15 correct places. This is exceptional accuracy: if the distance between the Earth and the Sun were known this accurately it would be within 0.006 inches of the correct value. The Babylonians seem to have been very clever.

What’s the square root of 10000? If the number of iterations is kept at 6, then the answer is a very poor one indeed: 323.1. Why? Some numbers (large ones) need more iterations than others. To guarantee that a good estimate of the square root is returned, an estimate of the error should be used. When the error is small enough, then the value will be good enough. The error will be x-yi*yi. The function should not loop a fixed number of times, but instead should repeat until the error is less than, say, 0.0000001. This function will be named roote, where the “e” is for “error.”

# Computer the square root of X to 7 decimal places

def roote (x):

y = x

# y is supposed to be the square root of x, so

e = abs(x-y*y)

# the error is x – y*y

while e > 0.0000001:

# repeat while the error is bigger

# than 0.0000001

y = (y + x/y)/2.0

# New estimate for square root

e = abs(x-y*y)

New error value

return y

This function will return the square root of any positive value of x to within 7 decimal places. It should check for negative values, though.

Parameters

A parameter can be either a name, meaning that it is a Python variable (object) of some kind, or an expression, meaning it has a value but no permanence in that it can’t be accessed later on—it has no name. Both are passed to a function as an object reference. The expression is evaluated before being given to the function and its type does not matter in so far as Python will always know what it is; its value is assigned a name when it is passed. Consider, for example, the function square in the following context:

...

pi = 3.14159

r = 2.54

c = square (2*pi*r)

print ("Circumference is ", c)

The assignments to pi and r are performed, and when the call to square occurs, the expression 2*pi*r is evaluated first. Its value is assigned to a temporary variable, which is passed as the parameter to square. Inside the function this parameter is named x, and the function calculates x squared and returns it as a value. It is as if the following code executes:

pi = 3.14159

r = 2.54

# call square(2*pi*r)

parameter1 = 2*pi*r     # set the parameter value

x = parameter1     # First parameter is named x inside SQUARE

returnvalue = x*x     # Code within SQUARE, return x*x

c = returnvalue     # assign result of function call to c

print ("Circumference is ", c)

This is not how a function is implemented, but shows how the parameter is effectively passed; a copy is made of the parameters and those are passed. If the expression 2*pi*r was changed to a simple variable, then the internal location of that variable would be passed.

Passing more structured objects works the same way but can behave differently. If a list is passed to a function, then the list itself cannot be modified, but the contents of the list can be. The list is assigned another name, but it is the same list. To be clear, consider a simple function that edits a list by adding a new element to the end:

def addend (arg):

arg.append("End")

z = ["Start", "Add", "Multiply"]

print (1, z)

addend(z)

print (1, z)

The list associated with the variable z is changed by this function call. It now ends with the string “End.” Output from this is:

1 [‘Start’, ‘Add’, ‘Multiply’]

2 [‘Start’, ‘Add’, ‘Multiply’, ‘End’]

Why is this? Because the name z refers to a thing that consists of many other parts. The name z is used to access them, and the function can’t modify the value of z itself. It can modify what z indicates; that is, the components. Think of it, if it makes it simpler, as a level of indirection. A book can be exchanged between two people. The receiver writes a note in it and gives it back. It’s the same book, but the contents are now different.

A small modification to addend() illustrates some confusing behavior. Instead of using append to add “End” to the list, use the concatenation operator “+”:

def addend (arg):

arg = arg + ["End"]

z = ["Start", "Add", "Multiply"]

print (1, z)

addend(z)

print (2, z)

Now the output is:

1 [‘Start’, ‘Add’, ‘Multiply’]

2 [‘Start’, ‘Add’, ‘Multiply’]

The component “End” is not a part of the list z anymore. It was made a component inside of the function, but it’s not present after the function returns. This is because the statement:

arg = arg + ["End"]

actually creates a new list with “End” as the final component, and then assigns that new list as a value to arg. This represents an attempt to change the value that was passed, which can’t happen: changing the value of arg will not change the value of the passed variable z. So, within the function arg is a new list with “End” as the final component. Outside, the list z has not changed.

The way that Python passes parameters is the subject of a lot of discussion on Internet blogs and lists. There are many names given for the method used, and while the technique is understood, it does differ from the way parameters are passed in other languages and is confusing to people who learned another language like Java or C before Python. The thing to remember is that the actual value of the thing (an object reference) being passed can’t be assigned a new value inside the function, but the things that it references or points to can be modified.

Default Parameters

It is possible to specify a value for a parameter in the instance that it is not given one by the caller. That may not seem to make sense, but the implication is that it will sometimes be passed explicitly and sometimes not. When debugging code it is common to embed print statements in specific places to show that the program has reached that point. Sometimes it is important to print out a variable or value there, other times it’s just to indicate that the program got to that statement safely. Consider a function named gothere:

def gothere (count, value):

print ("Got Here: ",count, " value is ", value)

then throughout the program, calls to gothere would be sprinkled with a different value for count every time; the value of count indicates the statement that has been reached. This is a way of instrumenting the program, and can be very useful for finding errors. So the code being debugged may look like:

year = 2015 # The code below is not especially meaningful

a = year % 19 # and is an example only.

gothere(1, 0)

b = year // 100

c = year % 100

gothere (2, 0)

d = (19*a+b-b//4-((b-(b + 8)//25 + 1)//3)+15)%30

e = (32+2 * (b % 4) + 2 * (c // 4) - d - (c % 4)) % 7

f = d + e - 7 * ((a + 11 * d + 22 * e) // 451) + 114

gothere (3, f)

month = f // 31

day = f % 31 + 1

gothere(4, day)

return date(year, month, day)

Output is:

Got Here: 1 value is 0

Got Here: 2 value is 0

Got Here: 3 value is 128

Got Here: 4 value is 5

2015 4 5

The program reaches each of the four checkpoints and prints a proper message. The first two calls to gothere did not need to print a value, only the count number. The second parameter could be given a default value, perhaps None, and then it would not have to be passed. The definition of the function would now be:

def gothere (count, value=None):

if value:

print ("Got Here: ",count, " value is ", value)

else:

print (Got Here: ", count)

and the output this time is:

Got Here: 1

Got Here: 2

Got Here: 3 value is 128

Got Here: 4 value is 5

2015 4 5

The assignment within the parameter list gives the name value a special property. It has a default value. If the parameter is not passed, then it takes that value; otherwise it behaves normally. This also means that gothere can be called with one or two parameters, which can be very handy. It is important to note that the parameters that are given a default value must be defined after the ones that are not. That’s because otherwise it would not be clear what was being passed. Consider the (illegal) definition:

def wrong (a=1, b, c=12):

. . .

Now call wrong with two parameters:

wrong (2,5)

What parameters are being passed? Is it a and b? Is it a and c? It is impossible to tell. A legal definition would be:

def right (b, a=1, c=12)

This function can be called as

right (19)

in which case b=19, a=1, and c=12. It can be called as:

right (19, 20)

in which case b=19, a=19, and c=12. It can be called as:

right (19, 19, 19)

in which case b=19, a=19, and c=19. But how can it be called passing b and c but not a? Like this:

right (19, c=19)

In this case a has been allowed to default. The only way to pass c without also passing a is to give its name explicitly so that the call is not ambiguous.

None

The value of the name None is something that has no value. It’s like null or nil in other languages, but is more general. For example, a function that is not explicitly assigned a return value will return None.

None has its own type (NoneType), and is used to indicate something that has no defined value or the absence of a value. It can be explicitly assigned to variables, printed, returned from a function, and tested. Testing for this value can be done using:

if x == None:

or by:

if x is None:

Example: The Game of Sticks

This is a relatively simple combinatorial game that involves removing sticks or chips from a pile. There are two players, and the game begins with a pile of 21 sticks. The first player begins by removing 1, 2, or 3 sticks from the pile. Then the next player removes some sticks, again 1, 2, or 3 of them. Players alternate in this way. The player who removes the last stick wins the game; in other words, if you can’t move, you lose.

Functions are useful in the implementation of this game because both players do similar things. The action connected with making a move, displaying the current position, and so on are the same for the human player and the computer opponent. The current status or state of the game is simply a number, the number of sticks remaining in the pile. When that number is zero then the game is over, and the loser is whatever player is supposed to move next. The code for a pair of moves, one from the human and one from the computer, might be coded in Python as follows:

displayState(val) # Show the game board

userMove = getMove() # Ask user for their move

val = val – userMove # Make the move

print ("You took ", userMove, " sticks leaving ", val)

if gameOver(val):

print("You win!")

else:

move = makeComputerMove (val) # Calculate the

computer's move

print ("Computer took ", move, " sticks leaving ", val)

if gameOver(val):

print("Computer wins!")

The current state of the game is displayed first, and then the human player is asked for their move. The move is simply the number of sticks to remove. When the move has been made, if there are no sticks left then the human wins. Otherwise, the computer calculates and makes a move; again, if no sticks remain then the game is over, in this case the computer being the winner. This entire section of code needs to be repeated until the game is over, of course.

There are four functions that must be written for this version: displayState(), getMove(), gameOver(), and makeComputerMove().

The function displayState() prints the current situation in the game. Specifically, it prints one ‘O’ character for each stick still in the pile, and does so in rows of 6. At the beginning of the game this function would print:

O O O O O O

O O O O O O

O O O O O O

O O O

which is 21 sticks. The code is:

def displayState(val):

k = val      # K represents the number of sticks

not printed

while k > 0:     # So long as some are not printed . . .

if k >=6:     # If there is a whole row, print it.

print ("O O O O O O ", end="")

k = k – 6      # Six fewer sticks are unprinted

else:

for j in range(0,k):     # Print the remainder

print ("O ", end="")

k = 0      # None remain

print ("")

This should be obvious. Also note that the function is named for what it does, and it does only one thing; it modifies no values outside of the function, and it serves a purpose that is needed multiple times. These are all good properties of a function.

The function getMove() will print a prompt to the user/player asking for the number of sticks they wish to remove and reads that value from the keyboard, returning it as the function value. Again, this function is named for what it does and performs a single, simple task. One possibility for the code is:

def getMove ():

n = int(input ("Your move: Take away how many? "))

while n<=0 or n>3:

print ("Sorry, you must take 1, 2, or 3 sticks.")

n = int(input ("Your move: Take away how many? "))

return n

The function gameOver() is trivial, but lends structure to the program. All it does is test to see whether the value of val, the game state variable, is zero. It leaves open the idea that there may be other end-of-game indicators that could be tested here.

def gameOver (state):

if state == 0:

return True

return False

Finally, the most complicated function, getComputerMove(), can be attempted. Naturally a good game presents a challenge to the player, and so the computer should win the game it if can. It should not play randomly if that is possible. In the case of this particular game, the winning strategy is easy to code. The player to make the final move wins, so if there are 1, 2, or 3 sticks left at the end, the computer would take them all and win. Forcing the human player to have 4 sticks makes this happen. The same is true if the computer can give the human player (i.e., leave the game in the state having) 8, 12, or 16 sticks (Check this!). So, if the human moves first (as it does in this implementation), the computer tries to leave the game in a state where there are 16, 12, 8, or 4 sticks left after its move. The code could be:

def getComputerMove (val):

n = val % 4

if n<=0:

return 1

else:

return n

There are a couple of details needed to finish this game properly that are left as an exercise.

Scope

A variable that is defined (first used) in the main program is called a global variable, and can be accessed by all functions if they ask for it. A variable that is used in a function can be accessed by that function, and is not available in the main program. It’s called a local variable. This scheme is called scoping: the locations in a program where a variable can be accessed is called its scope. It’s all pretty clear unless a global variable has the same name as a local one, in which case the question is: “What value is represented by this name?” If a variable named “x” is global and a function also declares a variable having the same name, this is called aliasing, and it can be a problem.

In Python a variable is assumed to be local unless the programmer specifically says it is global. This is done in a statement; for example:

global a, b, c

tells Python that the variables named a, b, and c are global variables and are defined outside of the function. This means that after the function has completed execution, those variables can still be accessed by the main program and by any other functions that declare them to be global.

Global variables are thought by some programmers to be a bad thing, but in fact they can be quite useful and can assist in the generality of the functions that are a part of the program. A global variable should represent something that is, in fact, global, something that should be known to the whole program. For instance, if the program is one that plays checkers or chess, then the board can be global. There is only one board, and it is essential to the whole program. The same applies to any program that has a central set of data that many of the functions need to modify.

An example of central data is game state in a video game. In the Sticks game program, for example, the function getComputerMove() takes a parameter—the game state. There is only one game state, and although for some games it can involve many values, in this case there is only one value: the number of sticks remaining. The function can be rewritten to use the game state variable val as a global in the following way:

def getComputerMove ():

global val

n = val % 4

if n<=0:

return 1

else:

return n

Similarly, the function that determines whether the game is over could use val as a global variable. On the other hand it would be poor stylistic form to have getMove() to use a global for the user’s move. The name does imply that the function will get a move, and so that value should be returned as an explicit function return value.

If a variable is named as global then that name cannot be used in the function as a local variable as well. It would be impossible to access it and would be confusing. It is a common programming error to forget to declare a variable as global. When this happens the variable is a new one local to the function, and starts out with a value of 0. Thus no syntax error is detected, but the calculation will almost certainly be incorrect. It might be a good idea to identify global variables in their name. For example, place the string “_g” at the end of the names of all globals. The game state above would be named val_g for example. This will be a reminder to declare them properly within functions.

Other kinds of data that could be kept globally would include lists of names, environment or configuration variables, complex data structures that represent a single underlying process, and other programming objects that are referred to as singletons in software engineering. In Python, because they have to be explicitly named in a declaration there is a constant reminder of the variable’s scope.

Variable Parameter Lists

The print() function is interesting because it seems to be able to accept any number of parameters and deal with them. The statement:

print(i)

prints the value of the variable i, and

print (i,j,k)

prints the value of all three variables i, j, and k. Is this some sort of special thing reserved for print() because Python knows about it? Nope. Any function can do this. Consider a function:

fprint ( "format string", variable list)

where the format string can contain the characters “f” or “i” in any combination. Each instance of a letter should correspond to a variable passed to the function in the variable list, and it will be printed as a floating point if the corresponding character in the format string is “f” and as an integer if it is “i.” The call:

fprint("fi", 12, 13)

will print the values 12 and 13 as a float and an integer respectively. How can this be written as a Python function?

The function would start out with the following definition:

def fprint (fstring, *vlist)

The expression *vlist represents a set of positional parameters, any number of them. This is preceded by a specific parameter fstring, which will be the format string. A simple test of this would be to just print the variables in the list to see if it works:

def fprint (fstring, *vlist)

for v in vlist:

print v

When called as fprint(“”, 12, 13, 14, 15) this prints:

12

13

14

15

It removes some of the magic to point out that what is going on is that the list of variable after the * character is turned into a tuple which is passed as the parameter, so the *vlist actually counts as a single parameter with many components. No magic.

To finish the original function, what has to be done is to peel characters off of the front of the format string, match them against a variable, and print the result as the format character dictates. So use the same loop as above, but also an index into the format string increases each time through and is used to indicate the format. It is also important that the number of format items equals the number of variables:

def fprint (s, *vlist):

i = 0

if len(s) != len(vlist): # Format string and variable

list agree?

print ("There must be the same number of

variables as format items.")

return

for v in vlist: # For each variable

if s[i] == "f": # Is the corresponding format

'f'?

fv = float(v) # Yes. Make it a float

print (fv, " ", end="") # . . . and print it

elif s[i] == "i": # Is the corresponding format

'i'?

iv = int(v) # Yes. Make it an integer

print(iv, " ", end="") # . . . and print it

else:

print ("?", end="") # Don't know what

this is. Print it

i = i + 1

All of the known positional parameters must come before the variable list; otherwise, the end of the variable list can’t be determined. There is a second complication, that being the existence of named parameters. Those are indicated by a parameter such as **nlist. The two “*” characters indicate a list of named variables. This is properly a more advanced topic.

Variables as Functions

Because Python is effectively untyped and variables can represent any kind of thing at all, a variable can be made to refer to a function; not the function name itself, which always refers to a specific function, but a variable that can be made to refer to any function. Consider the following functions, each of which does one trivial thing:

def print0():

print ("Zero")

def print1():

print ("One")

def print2():

print ("Two")

def print3():

print("Three")

Now make a variable reference one of these functions by means of an assignment statement:

printNum = print1      # Note that there is no parameter list

given

The variable printNum now represents a function, and when invoked the function it represents will be invoked. So:

printNum()

will result in the output:

One

Why did the statement printNum = print1 not result in the function print1 being called? Because the parameter list was absent. The statement:

printNum = print1()

results in a call to print1 at that moment, and the value of the variable printNum will be the return value of the function. This is the essential syntactic difference: print1 is a function value, and print1() is a call to the function. To emphasize this point, here is some code that would allow the English name of a number between 1 and 3 to be printed:

if a == 1:

printNum = print1      # Assign the function print1 to printNum

elif a == 2:

printNum = print2      # Assign the function print2 to printNum

else:

printNum = print3      # Assign the function print3 to printNum

. . .

printNum()      # Call the function represented by printNum

There are more subtle uses in this case. Consider this use of a list:

a = 1

printList = [print0, print1, print2, print3]

printNum = printList[a]

printNum()

will result in the output:

One

The final iteration of this is to call the function directly from the list:

printList[1]()

This works because printList[1] is a function, and a function call is a function followed by (). Seems overly complicated, doesn’t it? It is rarely used.

For those with an interest or need for mathematics, consider a function that computes the derivative or integral of another function. Passing the function to be differentiated or integrated as a parameter may be the best way to proceed in these cases.

Example: Find the Maximum Value of a Function

Maximizing a function can have important consequences in real life. The function may represent how much money will be made by manufacturing various objects, how many patients can get through an emergency ward in an hour, or how much food will be grown with a particular crop. If the function is

Why? well-behaved, then there are many mathematically sound ways to find a maximum or minimum value, but if a function is harder to deal with, then less analytical methods may have to be used. This problem proposes a search for the best pair of parameters to a problem that could be solved using a method called linear programming.

The problem goes like this:

A calculator company produces a scientific calculator and a graphing calculator. Long-term projections indicate an expected demand of at least 100 scientific and 80 graphing calculators each day. Because of limitations on production capacity, no more than 200 scientific and 170 graphing calculators can be made daily. To satisfy a shipping contract, a total of at least 200 calculators much be shipped each day.

If each scientific calculator sold results in a $2 loss, but each graphing calculator produces a $5 profit, how many of each type should be made daily to maximize net profits?

Let s be the number of scientific calculators manufactured and g be the number of graphing calculators. From the problem statement:

100 <= s <= 200

80 <= g <= 170

Also:

s + g > 200, or g > 200 - s

Finally, the profit, which is to be maximized, is:

P = –2s + 5g

First, code the profit as a function:

def profit (s, g):

return -2*s + 5*g

A search through the range of possibilities will run through all possible values of s and all possible values of g; that is, s from 100 to 200 and g from 80 to 170. The function will be evaluated at each point and the maximum will be remembered:

# Range for s is x0 .. x1

# Range for g is y0 .. y1

# s+g must be >= sum

def searchmax (f, x0, y0, x1, y1, sum):

pmax = -1.0e12

ps = -100

pg = -100

for s in range (x0, x1+1):      # For all possible s

for g in range (y0, y1+1):     # For all possible g

if s+g >= sum:      # Condition is ok?

p = f (s, g)      # Calculate the profit.

if p>=pmax:      # Best so far?

pmax = p      # Yes.

ps = s      # Save it and

pg = g      # the parameters

return ( (ps, pg) )

Finally, the call that does the optimization calls the search function passing the profit function as a parameter:

c = searchmax (profit, 100, 80, 200, 170, 200)

print (c)

The answer found is the tuple (100, 170), or s=100 and g = 170, which agrees with the correct answer as found by other methods. This is only one example of the value of being able to pass functions as parameters. Most of the code that does this is mathematical, but it may accomplish practical tasks like optimizing performance, drawing graphs and charts, and simulating real-world events.

Functions as Return Values

Just as any value, including a function, can be stored in a variable, any value, including a function, can be returned by a function. If a function that prints an English name of a number is desired, it could be returned by a function:

def print0():

print ("Zero")

def print1():

print ("One")

def print2():

print ("Two")

def print3():

print("Three")

def getPrintFun (a):     # Return a function to print a numeric

value 0..3

if a == 0:

return print0     # Return the function print0 as the result

elif a == 1:

return print1     # Return the function print1 as the result

elif a == 2:

return print2     # Return the function print2 as the result

else:

return print3     # Return the function print3 as the result

Calling this function and assigning it to a variable means returning a function that can print a numerical value:

printNum = getPrintFun(2) # Assign a function to printNum

and then:

printNum() # Call the function represented by printNum

results in the output:

Two

The function printFun returns, as a value, the function to be called to print that particular number. Returning the name of the function returns something that can be called.

Why would any of these seemingly odd aspects of Python be useful? Allowing a general case, permitting the most liberal interpretation of the language, would permit unanticipated applications, of course. And the ability to use a function as a variable value and a return result are a natural consequence of Python having no specific type connected with a variable at compilation time. There are many specific reasons to use functions in this way, on the other hand. Imagine a function that plots a graph. Being able to pass this function another function to be plotted is surely the most general way to accomplish its task.

Recursion

Python functions can be recursive. Recursion refers to a way of defining things and a programming technique, not a general language feature. Something that is recursive is defined at least partly in terms of itself. When talking about functions, a function is recursive if it contains within it a call to itself. This is normally done only when the thing that it is attempting to accomplish has a definition that is recursive. Recursion as a programming technique is an attempt to make the solution simpler. If it does not, then it is inappropriate to use recursion.

Each call to a function can be thought of as an instance of that function, and it will create all of the local variables that are declared within it. Each instance has its own copy of these, including its parameters, and each call returns to the caller as occurs with any other function call.

One important use of recursion is in reducing a problem into smaller parts, each of which has a simpler solution than does the whole problem. An example of this is searching a list for an item. If names = [Adams, Alira, Attenbourough, . . .] is a Python list of names in alphabetical order, answer the question: “Does the name Parker appear in this list?” Of course these is a built-in function that will do this, but this example is a pedagogical moment, and anyway perhaps the built-in function is slower than the solution that will be devised here.

The function will return True or False when passed a list and a name. The obvious way to solve the problem is to iterate through the list, looking at all of the elements until the name being searched for is either found or it is not possible to find it anymore (i.e., the current name in the list is larger than the target name). Another, less obvious way to conduct the search is to divide the list in half, and only search the half that has the target name in it. Consider the following names in the list:

. . . Broadbent Butterworth Cait Cara Carling Devers Dillan Eberly Foxworthy . . .

The name in the middle if this list is Carling. If the name being searched for is lexicographically smaller than Carling, then it must appear in the first half; otherwise, it must appear in the second half. That is, if it is there at all. A recursive example of an implementation of this is:

# Search the list for the given name, recursively.

def searchr (name, nameList):

n = len(nameList)           # How many elements in this list?

m = n/2

if name < nameList[m]:      # target name is in the first half

return searchr (name, nameList[0:m])

# Search the first half

elif name > nameList[m]:     # target must be in the second half

return searchr (name, nameList[m:n]

# Search the second half

else:

return True

If the name is in the list, this works fine. One way to think of this is that the function searchr() will take a string and a list as parameters and find the name in the list if it’s there. The way it works is not clear from outside the function (without being able to see the source) and should not matter. So, if the target is to be found in the first half of the list, for example, then call searchr() with the first half of the list.

searchr (name, nameList[0:m])

The fact that the call is recursive is not really the concern of the programmer, but it is the concern of the person who created the Python system. Now, how can the problem of a name not being in the list be solved?

When the name is not in the list, the program will continue until there is but one item in the list. If that item is not the target, then it is not to be found. So, if n=1 (only one item in the list) and nameList[0] is not equal to the target, then the target is not to be found in the list and the return value should be False. The final program will therefore be:

def searchr (name, nameList):

n = len(nameList)      # How many elements in this list?

m = int(n/2)

if n==1 and nameList[0]!=name:     # End of the recursive

calls

return False      # It's not in this list.

if name < nameList[m]: # target name is in the first half

return searchr (name, nameList[0:m])

# Search the first half

elif name > nameList[m]:

# target must be in the second half

return searchr (name, nameList[m:n])

# Search the second half

else:

return True

Many algorithms have fundamentally recursive implementations, meaning that the effective solution in code involves a recursive function call. Examples of very useful recursive functions will be examined in later chapters.

Creating a Python Module

In some of the examples given so far there is a statement at the beginning that looks like “import name.” The implication is that there are some functions that are needed by the program that are provided elsewhere, possibly by the Python system itself or perhaps by some other software developer. The idea of writing functions that can be reused in a straightforward way is very important to the software development process. It means that no programmer is really alone, that code is available for doing things like generating random numbers or interfacing with the operating system or the Internet, and that it does not need to be created each time. In addition, there is an assumption that a module works correctly. When a programmer builds a collection of code for their own use, it needs to be tested as thoroughly as possible, and from that time on it can be used in a package with confidence. If a program has errors in it, then look in the code for that program first and not in the modules. This makes debugging code faster.

What is a module? It is simply a function or collection of functions that reside in a file whose name ends in “.py.” Technically, all of the code developed so far qualifies as modules. Consider as an example the function from the previous section that finds the maximum value in a list. Save the functions max() and maxr() in a file named max.py. Now create a new Python program named usemax.py and place it in the same directory as max.py. If the two files are in the same directory, then they can “see” each other in some sense.

Here is some code to place in the file usemax.py:

import max

d = [12,32,76,45,9,26,84,25,61, 66, 1,2]

print ("MAX is ", max.max(d), " MAXR is ", max.maxr(d))

if max.maxr(d) != max.max(d):

print ("*** NOT EQUAL ****")

This program is just a test of the two functions to make certain that they return the same value for the same list, the variable d. Note two things:

1)The statement import max occurs at the beginning of the program, meaning that the code inside this file is available to this program. Python will look inside of this file for function and variable names.

2)When the function max() or maxr() is called, the function name is preceded by the module name (max) and a period. This syntax informs the Python system that the name maxr() (for example) is found in the module max and not elsewhere.

The first time that the module is loaded into the Python program, the code in the module is executed. This allows any variable initializations to be performed. Henceforth that code is not executed again, and functions within the module can be called knowing that the initializations have been performed.

The module could reside in the same directory as the program that uses it, but it does not have to. The Python system recognizes a set of directories and paths, and modules can be placed in some of those locations as well, making it easier for other programs on the same computer to take advantage of them. On the computer used to create the examples in this book, the directory C:\Python34\Lib can be used to store modules, and they will be recognized by import statements.

Finally, if the syntax max.maxr(list) seems a bit cumbersome, then it is possible to import specific names from the module into the program. Consider the following rewrite of usemax.py:

from max import max, maxr

d = [12,32,76,45,9,26,84,25,61, 66, 1,2]

print ("MAX is ", max(d), " MAXR is ", maxr(d))

if maxr(d) != max(d):

print ("*** NOT EQUAL ****")

The statement from max import max, maxr instructs Python to recognize the names max and maxr as belonging to the module named max (i.e., as residing in the file named max.py). In that case the function can be called by simply referencing their names.

CHAPTER 5

FILES: INPUT AND OUTPUT

The first thing to know about a file is that it is a collection of bytes stored on a disk or similar device. One set of bytes can look very much like another, and unless the format of the file (i.e., the way the bytes are ordered) and its basic contents (i.e., what kind of thing the bytes represent) is known ahead of time, the information stored there is unusable. Computer programs are written assuming that the files they will read have a particular nature; if given a file that does not have that nature, the program will not function properly.

What kinds of files are there? Here is a short list:

Text files. These contain characters that a person can read, and can be thought of as documents.

Executable files. These hold instructions that a computer can execute. Such a file is a program or an “app.”

Data files. It could also be a text file if it is stored as characters, but it could be a set of bytes that represents integers or real numbers.

Image files. There are many types of image files, and they contain pictures in digital format. Many digital cameras use a format called JPEG, but GIF or PNG are two of many others. Not only are images stored in such a file, but also data about how large the image is, when it was taken, and other details.

Sound files. The more common sound file is the MP3, but there are many others.

Video. MPEG and AVI are standard formats for video, and there are a great many files of this sort available on the Internet.

Web pages. These are a special kind of text file. They can be examined and modified using basic text editors, but can’t be viewed properly (i.e., as a web page) except through a browser, which is really a special kind of display utility that can both draw images and connect to the Internet to download more information.

All of these files, and indeed all files, have certain things in common. Some of these things can be ignored when writing Python programs, but others cannot.

Files have names. The first way to access a file is usually by specifying its name.

Files have a size. It is usually expressed in bytes, which is to say characters.

Basic operations on a file are read and write. To read from a file means to examine a byte (at least). Writing is the reverse process: a byte or bytes are copied from memory onto disk.

Files must be open before they can be used. To open a file a program must know its name, and then invoke the open function or program. The open function and many other file-related operations belong to the operating system of the computer, and not normally to the language. It’s one reason why so much software is not portable.

Only one program at a time can write to a file. Many programs can read a file simultaneously, but only one can write to it, and not while anyone else is reading it.

Another thing to consider is that text, and therefore text files, are a principal means for communication between humans and computers. It is critical that any scheme for writing text to a file takes into account the human aspects of text: sentences, lines, paragraphs, special characters, numbers, and so on. This chapter will be concerned with the way in which Python can use files, with files as a concept in general, and with how humans think of data and files.

How long does it take to access a block of data on the disk? The time to access a random data item can be estimated as 14.15 milliseconds. Disk is vastly slower than memory, and in order to use the data, it must be copied into memory. This is a bottleneck in many computer systems.

Problem: Read a Number from the Keyboard and Divide It by 2

In this instance the problem is one of type: how to treat integers like integers and floats like floats. When the string s is read in it’s just a string, and it is supposed to contain an integer. However, users will be users, and some may type in a float by mistake. The program should not crash just because of a simple inputting mistake. How is this situation handled?

The problem is that when the string is converted into an integer, if there is a decimal point or other non-digit character that does not belong then an error will occur. It seems that an answer would be to put the conversion into a try statement block and if the string has a decimal point then convert the string to float within the except part. Something like this:

s = input("Input an integer: ")

try:

k = int(s)

ks = k//2

except:

z = float(s)

k = int(z/2)

print (k)

If the user types “12” in response to the prompt “Input an integer: ” then the program prints “6.” If the user types “12.5” then the program catches a ValueError, because 12.5 is not a legal integer. The except part is executed, converting the number to floating point, dividing by 2, then finally converting to an integer.

One problem is that the except part is not part of the try, so errors that happen there will not be caught. Imagine that the user types “one” in response to the prompt. The call to int(s) results in a ValueError, and the except part is executed. The statement:

z = float(s)

will result in another ValueError. This one will not be caught and the program will stop executing, giving a message like:

ValueError: could not convert string to float: 'one'

s = input("Input an integer: ")

try:

k = int(s)

k = k//2

except ValueError:

try:

z = float(s)

k = int(z/2)

except ValueError:

k = 0

print (s, k)

Using Files in Python

The general paradigm for reading and writing files is the same in Python as it is in most other languages. The steps for reading or writing a file are these:

1)Open the file. This involves calling a function, usually named open, and passing the name of the file to be used. Sometimes the mode for opening is passed; that is, a file can be opened for input, output, update (both input and output), and in binary modes. The function locates the file using the name and returns a variable that keeps track of the current state of input from the file. A special case exists if there is no file having the given name.

2)Read data from the file. Using the variable returned by open, a function is called to read data. The function might read a character, or a number, or a line, or the whole file. The function is often called read, and can be called multiple times. The next call to read will read from where the last call ended. A special case exists when all of the data has been read from the file (Called the end of file condition).

OR

2)Write data to the file. Using the variable returned by open, a function is called to write data to the file. The function might write a character, or a number, or a line, or many lines. The function is often called write, and can be called multiple times. The next call to write will continue writing data from where the last call ended. Writing data most frequently appends data to the end of the file.

3)Close the file. Closing a file is also accomplished using a call to a function (yes, it is usually named close). This function frees storage associated with the input process and in some cases unlocks the file so it can be used by other programs. A variable returned by open is passed to close, and afterwards that variable can’t be used for input anymore. The file is no longer open.

Open a File

Python provides a function named open that will open a file and return a value that can be used to read from or write to the file. That value actually refers to a complex collection of values that refers to the file status and is called a handle or a file descriptor in the computing literature, although knowledge of the details is not needed to use it. It can be thought of as something of type file, and must be assigned to a variable or the file can’t be accessed. The open function is given the name of the file to be opened and a flag that indicates whether the file is to be read from or written to. Both of these are strings. A simple example of a call to open is:

infile = open ("datafile.txt", "r")

This will open a file named “datafile.txt” that resides in the same directory as does the Python program, and opens it for input: the “r” flag means read. It returns the handle to the variable infile, which can now be used to read data from the file.

There are some details that are crucial. The name of the file on most computer systems can be a path name, which is to say the name including all directory names that are used to find it on your computer. For example, on some computers the name “datafile.txt” might have the complete path name “C:/parker/introProgramming/

chapter05/datafile.txt.” If path names are used, the file can be opened from any directory on the computer. This is handy for large data sets that are used by multiple programs, such as names of customers or suppliers.

The read flag “r” that is the second parameter is what was called the mode in the previous discussion. The “r” flag means that the file will be open for reading only, and starts reading at the beginning of the file. The default is to read characters from the file, which is presumed to be a text file. Opening with the mode “rb” opens the file in binary format, and allows reading non-text files, such as MP3 and video files.

Passing the mode “w” means that the file is to be written to. If the file exists, then it will be overwritten; if not, the file will be created. Using “wb” means that a binary file is to be written.

Append mode is indicated by the mode parameter “a,” and it means that the file will be opened for writing, and if the file exists then writing will begin at the end of the existing file. In other words, the file will not start over as being empty but will be added to, at the end of the file. The mode “ab” appends data to a binary file. There are a few other modes that will be discussed when they are needed.

If the file does not exist and it is being opened for input, there is a problem. It’s an error, of course; a nonexistent file can’t be read from. There are ways to tell whether a file exists, and the error caused by a nonexistent file can be caught and handled from within Python. This involves an exception. It is always a bad idea to assume that everything works properly, and when dealing with files it is especially important to check for all likely problems.

File Not Found Exceptions

The proper way to open a file is within a try-except pair of statements. This will ensure that nonexistent files or permission errors are caught rather than causing the program to terminate. The basic scheme is simple:

try:

infile = open ("datafile.txt", "r")

except FileNotFoundError:

print ("There is no file named 'datafile.txt'. Please try again")

return     # end program or abort this section of code

The exception FileNotFoundError will be thrown if the file name can’t be found. What to do in that case depends on the program: if the file name was typed in by the user, then perhaps they should get another chance. In any case the file is not open, and data can’t be read.

There are multiple versions of Python on computers around the world, and some versions have different names for things. The examples here all use Python 3.4. In other versions the FileNotFoundError exception has another name; it may be IOError or even OSError. The documentation for the version being used should be consulted if a compilation error occurs when using exceptions and some built-in functions. For the 3.4 compiler version, all three seem to work with a missing file.

All attempts to open a file should take place while catching the FileNotFoundError exception.

Reading from Files

After a file is opened with a read mode, the file descriptor returned can be used to read data from the file. Using the variable infile returned from the call to open () above, a call to the method read() can get a character from the file:

s = infile.read(1)

Reading one character at a time is always good enough, but is inefficient. If a block on disk is 512 characters (bytes), then that should be a good number of bytes to read at one time, or a multiple of that. Reading more data than you need and saving it is called buffering, and buffers are used in many instances: live video and audio streaming, audio players, and even in programming language compilers. The idea is to read a larger block of data than is needed at the moment and to hand it out as needed. Reading a buffer could be done as:

s = infile.read(512)

and then dealing characters from the strings one at a time as needed. A buffer is a collection of memory locations that is temporary storage for data that was recently on secondary store.

Text files, those that contain printable characters that humans can read, are normally arranged as lines separated by a carriage return or a linefeed character, something usually called a newline. An entire line can be read using the readline() function:

s = infile.readline()

A line is not usually a sentence, so many lines might be needed to read one sentence, or perhaps only half of a line. Computer text files are structured so that humans can read them, but the structure of human language and convention is not understood by the computer nor it is built into the file structure. However, it is normal for people to make data files that contain data for a particular item or event on one line, followed by data for the next item. If this is true then one call to readline() will return all of the information for a particular thing.

End of File

When there are no more characters in the file, read() will return the empty string: “”. This is called the end of file condition, and it is important that it be detected. There are many ways to open and read files, but for reading characters in this way the end of file is checked as follows:

infile = open("data.txt", "r")

while True:

c = infile.read(1)

if c == '':

print ("End of file")

exit()

else:

c = infile.read(1)

When reading a file in a for statement, the end of file is handled automatically. In this case the loop runs from the first line to the final line and then stops.

for c in f:

print ("'", c, "'")

Oddly an exception can’t be used in an obvious way for handling the end of file on file input. However, when reading from the console using the input() function, the exception EOFError can be caught:

while True:

try:

c = input()

print (c)

except EOFError:

print ("Endfile")

break

There are many errors that could occur for any set of statements. It is possible to determine what specific exception has been thrown in the following manner:

while True:

try:

c = input()

print (c)

except Exception as x:

print (x)

break

This code prints “EOF when reading a line” when the end of file is encountered.

Common File Input Operations

There are a few common ways to use files that should be mentioned as patterns. Although one should never use a pattern if it is not understood, it’s sometimes handy to have a few simple snippets of code that are known to perform basic tasks correctly. For example, one common operation to use with files is to read each line from a file, followed by some processing step. This looks like:

f = open ("data.txt", "r")

for c in f:

print ("'", c, "'")

f.close()

The expression c in f results in consecutive lines being read from the files into a string variable c, and this stops when no more data can be read from the file.

Another way to do the same thing would be to use the readline() function:

f = open ("data.txt", "r")

c = f.readline()

while c != '':

print ("'", c, "'")

c = f.readline()

f.close()

In this case the end of file has to be determined explicitly, by checking the string value that was read to see if it is null.

CSV Files

A very common format for storing data is called Comma Separated Variable (CSV) format, named for the fact that each pair of data items have a comma between them. CSV files can be used directly by spreadsheets such as Excel and by a large collection of data analysis tools, so it is important to be able to read them correctly.

A simple CSV file named planets.txt is provided for experimenting with reading CSV files. It contains some basic data for the planets in Earth’s solar system, and while there is no actual standard for how CSV files must look, this one is typical of what is usually seen. The first line in the file contains headings for each of the variables or columns, separated by commas. This is followed by nine lines of data, one for each planet. It’s a small data file as these things are counted, but illustrative for the purpose. Here it is:

Problem: Print the Names of Planets Having Fewer than Ten Moons

This is not a very profound problem, and uses the raw data as it appears on the file. The file must be opened and then each line of data is read; the value of the 11th data element (i.e., index 10) is retrieved and compared against 10. If larger, the name of the planet (index 0) is printed. The plan is:

Open the file

Read (skip over) the header line

For each planet

Read a line as string s

Break s into components based on commas giving list P

If P[10] < 10 print the planet name, which is P[0]

It is all something that has been done before except for breaking the string into parts based on the comma. Fortunately the designers of Python anticipated this kind of problem and have provided a very useful function: split(). This function breaks up a string into parts using a specified delimiter character or string and returns a list in which each component is one section of the fractured string. For example:

s = "This is a string"

z = s.split(" ")

yields the list z = [“This”, “is”, “a”, “string”]. It splits the string s into substrings at each space character. A call like s.split(“,”) should give substrings that are separated by a comma. Given the above sketch and the split() function, the code now pretty much writes itself.

try:

# Open the file

infile = open ("planets.txt", "r")

# Read (skip over) the header line

s =infile.readline()

# For each planet

for i in range (0, 8):

# Read a line as string s

s = infile.readline()

# Break s into components based on commas giving list P

P = s.split (",")

# If P[10] < 10 print the planet name, which is P[0]

if int(P[10])<10:

print (P[0], " has fewer than 10 moons.")

except FileNotFoundError:

print ("There is no file named 'planets.txt'.

Please try again")

Things to notice: almost the entire program resides within a try statement, so that if the file does not exist, then a message will be printed and the program will end normally. Also note that P[10] has to be converted into an integer, because all components of the list P are strings. Strings are what has been read from the file.

CSV files are common enough so that Python provides a module for manipulating them. The module contains quite a large collection of material, and for the purposes of the planets.py program only the basics are needed. To avoid the details of a general package, a simpler version is included with this book: simpleCSV has the essentials needed to read most CSV files while being written in such a way that a beginning programmer should be able to read and understand it.

To use it, the simpleCSV module is first imported. This makes two important functions available: nextRecord() and getData(). The nextRecord() function reads one entire line of CSV data. It allows skipping lines without examining them in detail (like headers). The function getData() will parse one line of data, the last one read, into a tuple, each element of which is one of the comma separated fields.

The simpleCSV library needs to be in the same directory as the program that uses it, or be in the standard Python directory for installed modules. The source code resides on the accompanying disk and is called simpleCSV.py. The program above can be rewritten to use the simpleCSV module as follows:

import simpleCSV

try:                         # Read (skip over) the header line

# Open the file

infile = open ("planets.txt", "r")

simpleCSV.nextRecord(infile)     # Read the header

for i in range (0, 8):      # For each planet

simpleCSV.nextRecord(infile) # Read a line and collect substrings in a list

p = simpleCSV.getData(infile)

if int(P[10])<10:      # If number of moons less than 10

print (P[0], " has fewer than 10 moons.") # print the planet name

except FileNotFoundError:

print ("There is no file named 'planets.txt'. Please try again")

The With Statement

A difficulty with the code presented so far is that it does not clean up after itself. A file should be closed after input from it or output to it is finished; none of the programs written so far do that, at least not after the file operations are complete. There has been no significant discussion of the close() operation, but what it does has been described. Normally when a program terminates, its resources are returned to the system, including the closing of any open files. Intentionally closing a file is important for three reasons: first, if the program aborts for some reason, open files should be closed by the system but may not be, and file problems can be the result. Second, closing a file can be used as a step in reusing it. Opening it again starts reading it at the beginning. Third, closing a file frees its resources. Programs that use many files and/or many resources will profit from freeing them when they are no longer needed.

The Python with statement, in its simplest form, takes care of many of the details surrounding file access. An example of its use is:

try:

with open ("planets.txt") as infile: # Open the file

simpleCSV.nextRecord(infile) # Read the header

for i in range (0, 9): # For each planet

simpleCSV.nextRecord(infile) # Read a line, make a

list

P = simpleCSV.getData(infile)

if int(P[10])<10: # If number of moons

less than 10

print (P[0], " has fewer than 10 moons.")

# print the name

except FileNotFoundError:

print ("There is no file named 'planets.txt'.

Please try again")

Once the file is open, the with statement guarantees that certain errors will be dealt with and the file will be closed. The problem is that the file has to be open first, so the FileNotFound error should still be caught as an exception.

Writing to Files

The first step in writing to a file is opening it, but this time for output:

outfile = open ("out.txt", "w")

The “w” as the second parameter to open() means to open the file for writing. When writing to a file it is important to note that opening it will create a new file by default. If a file with the given name already exists, it will be rewritten, and the previous contents will be gone.

The basic file output function is write(); it takes a parameter, a string to be written to the file. It only writes strings, so numbers and other types have to be converted into strings before being written. Also, there is no concept of a line. This function simply moves characters to a file, one at a time, in the order given. In order to write a line, an end of line character has to be written. This is usually specified in a string as “\n,” spoken as “backslash n.” The “n” stands for newline.

Example: Write a Table of Squares to a File

This will illustrate the typical code involved in writing to a file. The file must be opened, then a loop from 0 to 25 is constructed. Each number in that range is written to the file, as is that number multiplied by itself. Each output string represents a line, and so must have a newline character added to the end.

outfile = open ("out.txt", "w")

outfile.write (" X            X squared \n")

for i in range (0, 25):

sout = " "+str(i)+" "+str(i*i)+"\n"

outfile.write (sout)

outfile.close()

Note that the integers are explicitly converted into strings and concatenated into a line to be written. The elements of the line could be written in separate calls to write:

outfile = open ("out.txt", "w")

outfile.write (" X            X squared \n")

for i in range (0, 25):

outfile.write (" ")

outfile.write (str(i))

outfile.write (" ")

outfile.write (str(i*i))

outfile.write ("\n")

outfile.close()

The output file is closed after all data has been written.

Another common file operation is to copy a file to another, character by character. A file is opened for input and another for output. The basic “read a file” pattern is used, with the addition of a file output after each character is read:

f = open ("data.txt", "r")

g = open ("copy.txt", "w")

c = f.read(1)

while c != '':

g.write(c)

c = f.readline(1)

f.close()

g.close()

Two files can be merged into a single file in many ways: one file after another, a line from one file followed by a line from another, character by character, and so on. A simple merging of two files where one is copied first followed by the other is:

f = open ("data1.txt", "r")

outfile = open ("copy.txt", "w")

c = f.read()

outfile.write(c)

f.close()

g = open ("data2.txt", "r")

c = g.read()

outfile.write(c)

g.close()

outfile.close()

Copying the input from console to a file means reading each line using input() and writing it to the file. This code assumes that an empty input line implies that the copying is complete.

outfile = open ("copy.txt", "w")

line = input ("! ")

while len(line)>1 or line[0]!="!":

outfile.write(line)

outfile.write ("\n")

line = input("! ")

outfile.close()

The end of the line is indicated by a character, which is represented by the string “\n.” Reading characters from a file will read the end of line character also, and detecting it can be very important.

f = open ("data.txt", "r")

c = f.read(1)

while c != '':

print ("'", c, "'")

c = f.read(1)

if c == '\n':

print ("Newline")

Appending Data to a File

Opening the file in “w” mode starts writing at the beginning of the file, and will result in existing data being lost. This is not always desirable. For example, what if a log file is being created? The log should contain a record of everything that has happened, not just the most recent thing.

Opening the file in append mode, signified by the parameter “a,” opens the file for output and starts writing at the end of the file if it already exists. This means that data can be added to the end of an existing file.

Example: Append Another 20 Squares to the Table of Squares File

The previous example created a file named “out.txt” and wrote 26 lines to it. It was a table of squares, and the final one was 24. This example will therefore begin at 25 and add 20 more values to the table.

The main difference is the opening of the output file in append mode, and starting the loop at 25 instead of at 0:

outfile = open ("out.txt", "a")

for i in range (25, 45):

sout = " "+str(i)+"              "+str(i*i)+"\n"

outfile.write (sout)

outfile.close()

The file “out.txt” will contain the squares of the integers between 0 and 44 inclusive after this program runs.

CHAPTER 6

CLASSES

A class, in the general sense, is a template for something that involves data and operations (functions). An object is an instance of a class, a specific instantiation of the template. Defining a class in Python involves specifying a class name and a collection of variables and functions that will belong to that class. The main class that has been referred to so far has only a few characteristics that we know about for certain.

Consider the joke that begins with the phrase “A man walks into a bar”? What is a man, what is a bar, and what does walking entail? Walking seems to be something that a man can do, an action they can perform. And a bar is a place where a man can walk. Can a man do anything else but walk? Is a bar the only place a man can walk to?

A man could be a class. It does have a function called walksInto, as one example. A first draft of the man class could be as follows:

class man:

def walksInto (aBar):

# code goes here

In the above example walksInto is a method; essentially, a method is any function that is part of a class.

Classes can have their own data too, which would be variables that “belong” to the class in that they exist inside it. Such variables can be used inside the class but can’t be seen from outside.

Looking closely at the simple class man above, notice that it is actually still a rather abstract thing. In the narrative about a man walking into a bar it was a specific man, as indicated by a variable aMan. So it would seem that a class is really a description of something, and that examples or instances should be created in order to make use of that description. This is correct. In fact, many individual instances of any class can be created (instantiated) and assigned to variables. To create a new instance of the class man, the following syntax could be used:

aMan = man()

When this is done all of the variables used in the definition of man are allocated. In fact, whenever a new man class is created, a special method that is local to man is called to initialize variables. This method is the constructor, and can take parameters that help in the initialization. Creating a man might involve giving him a name, so the instantiation may be:

aMan = man("Jim Parker")

In this case the constructor accepts a parameter, a string, and probably assigns it to a variable local to the class (Name, most likely). The constructor is always named __init__:

def __init__ (self, parameter1, parameter2, . . .):

The initial parameter named self is a reference to the class being defined. Any variable that is a part of this class is referred to by prefixing the variable name with “self.” To make a constructor for man that accepted a name, it would look like this:

def __init__ (self, name):

self.Name = name

When a man is created, the statement would be:

aMan = man ("Jim Parker")

This metaphor has fulfilled its purpose for the moment.

The Python Class – Syntax and Semantics

The man walks into a bar example illustrates many aspects of the Python class structure but obviously omits many details, especially formal ones that can be so important to a programmer. A class looks like a function in that there is a keyword, class, and a name and a colon, followed by an indented region of code. Everything in that indented region “belongs” to the class, and cannot be used from outside without using the class name or the name of a variable that is an instance of the class.

The method __init__ is used to initialize any variables that belong to the class. Java would call this method a constructor, and that’s how it will be referenced here too. Any variables that belong to the class must be accessed through either an instance (from outside of the class) or by using the name self (from within the class). So, self.name would refer to a variable that was defined inside of the class whereas simply using name would refer to a variable local to a method. When __init__ is called a set of parameters can be passed and used to initialize variables in the class. If the first parameter is self, it means that the method can access class-local variables, otherwise it cannot. Normally self is passed to __init__ or it can’t initialize things. Any variable initialized within __init__ and prefixed by self is a class-local variable. Any method that is passed self as a parameter can define a new class-local variable, but it makes sense to initialize all of them in one place if that’s possible.

A simple example of a class, initialization, and method is:

class person:

def __init__ (self, name):

self.name = name

def introduce (self):

print ("Hi, my name is ", self.name)

me = person("Jim")

me.introduce()

This class has two methods, __init__() and introduce(). After the class is defined, a variable named me is defined and is given a new instance of the person class having the name “Jim.” Then this variable is used to access the introduce method, which prints the introduction message “Hi, my name is Jim.” A second instance could be created and assigned to a second variable named you using:

you = person ("Mike")

and the method call

you.introduce()

would result in the message “Hi, my name is Mike.” Any number of instances can be created, and some may have the same name as others—they are still distinct instances.

A new class-local variable can be created by any method. In introduce(), for example, a new local named introductions can be created simply by assigning a value to it.

def introduce (self):

print ("Hi, my name is ", self.name)

self.introductions = True

This variable is True if the method introductions has been called. The main program can access this variable directly. If the main program becomes:

me = person("Jim")

me.introduce()

print (me.introductions)

then the program will generate the output:

Hi, my name is Jim

True

This is the essential information needed to define and use a class in Python. A more complex example would be useful in seeing how these features can be used in practice.

A Really Simple Class

A common example of a basic class is a point, a place on a plane specified by x and y coordinates. The beginning of this class is:

class point:

def __init__ (self, x, y):

self.x = x

self.y = y

This simply represents the data associated with a mathematical point. What more does it need? Well, two points have a distance between them. A distance method could be added to the point:

def distance (self, p):

d = (self.x-p.x)*(self.x-p.x)+(self.y-p.y)*(self.y-p.y)

return sqrt(d)

If a traditional function were to be used to compute distance, it would be written similarly but not identically. It would take two points as parameters:

def distance (p1, p2):

d = (p1.x-p2.x)*(p1.x-p2.x) + (p1.y-p2.y)* (p1.y-p2.y)

return sqrt(d)

The distance method uses one of the points as a preferred parameter, in a sense. The distance between points p1 and p2 would be calculated as:

d = p1.distance(p2) or d = p2.distance(p1)

using the distance method, but as:

d = distance (p1, p2)

if the function was used. To a degree the difference is a philosophical one. Is distance some property that a point has from another point (the method), or is it something that is a thing that is calculated for two things (the function)? A programmer begins, after a while, to see the methods and data of a class as belonging to the object, and as being somehow properties of it. That’s what makes a class a type definition.

Many object-oriented languages offer the concept of accessor methods. All that an accessor method does is return a value of importance to a user of a class. The x and y positions are variables local to the class, and many would agree that they should have an accessor method:

def getx (self):

return self.x

def gety (self):

return self.y

Rewriting the distance() method to use accessor methods changes it only slightly:

def distance (self, p):

d = (self.x-p.getx())*(self.x-p.getx()) +

(self.y-p.gety())* (self.y-p.gety())

return sqrt(d)

Methods called mutators or setters are used to modify the value of a variable in a class. They may do more than that, such as checking ranges and types, and tracking modifications.

def setx (self, x):

self.x = x

def sety (self, y):

self.y = y

There are other methods that could be added to even this simple class just in case they were needed, such as to draw the point, to return a string that describes the object, to rotate about the origin or some other point, a destructor (what to do when the object is no longer needed), and so on. Until it is known what the class will be used for there may not be any value for this effort, but if a class is being provided for general utility, like the Python string, as much functionality would be provided as the programmer’s imagination could invent. A draw method could simply print the coordinates, and could be useful for debugging:

def draw (self):

print ("(", self.x, ",", self.y, ") ")

Using this class involves creating instances and using the provided methods, and that should be all. A triangle consists of three points. A triangle class could be defined as:

class triangle:

def __init__ (self, p0, p1, p2):

self.v0 = p0

self.v1 = p1

self.v2 = p2

self.x = (p0.getx()+p1.getx()+p2.getx())/3

self.y = (p0.gety()+p1.gety()+p2.gety())/3

def set_vertices (self, p0, p1, p2):

self.v0 = p0

self.v1 = p1

self.v2 = p2

def get_vertices (self):

return ( (self.v0, self.v1, self.v2) )

def getx (self):

return self.x

def gety (self):

return self.y

The (x, y) value of a triangle is its center, or the average value of the x and the y coordinates of the vertices. These are the basic methods. A triangle is likely to be drawn somehow, and the next chapter will explain how to do that specifically. However, without knowing the details, a triangle is a set of lines drawn between the vertices and so might be done that way. As it is, using text only, it will print its vertices:

def draw (self):

print ("Triangle:")

self.v0.draw()

self.v1.draw()

self.v2.draw()

The triangle can be moved to a new position. A change in the x and y locations specifies the change, and it is done by changing the coordinates of each of the vertices:

def move (self, dx, dy)

coord = p0.getx()

p0.setx(coord+dx)

coord = p0.gety()

p0.sety(coord+dy)

coord = p1.getx()

p1.setx(coord+dx)

coord = p0.gety()

p1.sety(coord+dy)

coord = p2.getx()

p2.setx(coord+dx)

coord = p2.gety()

p2.sety(coord+dy)

self.x = self.x + dx

self.y = self.y + dy

In this way of expressing things, it is clear that moving the triangle is a matter of changing the coordinates of the vertices. If each point had a move() method, then it would be clearer: moving a triangle is a matter of moving each of the vertices:

def move (self, dx, dy):

p0.move(dx, dy)

p1.move(dx, dy)

p2.move(dx, dy)

self.x = self.x + dx

self.y = self.y + dy

Which of these two move() methods seems the best description of what is happening? The more complex are the classes, the more value there is in making an effort to design them to effectively communicate their behaviors and to make things easier to expand and modify. It is also plain that the move() method for a point is simpler than that for a triangle. That fact is invisible from outside the class, and it is actually not relevant.

Encapsulation

In the example of the point class, there is no actual need for an accessor method because the variables can be accessed from outside the class, in spite of the arguments that have been given for more controlled use of these variables. A careful programmer would want to ensure the integrity of classes by forcing the variables to remain protected in some way, and Python allows this while not requiring it.

The variables x and y are accessible and modifiable from outside because of how they are named. Any variable name in a class that begins with an underscore character (‘_’) cannot be modified by code that does not belong to the class. Such a variable is said to be protected. A variable name that begins with two underscore characters can’t be modified or even examined from outside of the class, and is said to be private. All other variables are public. This applies to method names too, so the method __init__() that is the usual constructor is private.

Rewriting the point class to make the internal variables private would be done like this:

class point:

def __init__ (self, x, y):

self.__x = x

self.__y = y

def getx (self):

return self.__x

def gety (self):

return self.__y

det setx (self, x):

self.__x = x

def sety (self, y):

self.__yy = y

def distance (self, p):

d = (self.__x-p.getx())*(self.__x-p.getx()) +

(self.__y-p.gety())* (self.__y-p.gety())

return sqrt(d)

def move(self, dx, dy):

self.__x = self.__x + dx

self.__y = self.__y + dy

def draw (self):

print ("(", self.__x, ",", self.__y, ") ")

Now the internal variables x and y can’t be modified or even have their values examined unless explicitly allowed by a method.

Example: A Deck of Cards

Traditional playing cards these days have red and black colors, four suits, and a total of 52 cards, 13 in each suit. Individual cards are components of a deck, and can be sorted: a 2 is less than a 3, a Jack less than a King, and so on. The Ace is a problem: sometimes it is the high card, sometimes the low card. A card would possess the characteristics suit and value. When playing card games cards are dealt from the deck into hands of some number of cards: thirteen cards for bridge, five for most poker games, and so on. The value of a card usually matters. Sometimes cards are compared against each other (poker), sometimes the sum of values is key (blackjack, cribbage), and sometimes the suit matters. These uses of a deck of cards can be used to define how classes will be created to implement card games on a computer.

Operations on a card could include to view it (it could be face up or face down) and to compare it against another card. Comparison operations could include a set of complex specifications to allow for aces being high or low and for some cards having special values (spades, baccarat) so a definition step might be very important.

A deck is a collection of cards. There is usually one of each card in a deck, but in some places (e.g., Las Vegas) there could be four or more complete decks used when playing Blackjack. Operations on a deck would include to shuffle, to replace the entire deck, and to deal a card or a hand. With these things in mind, a draft of some Python classes for implementing a card deck can be created:

class card:

def __init__ (self,

face, suit):

def value():

def suit():

def facevalue():

def view ():

def compare():

def initialize()

class deck:

def __init__ (self):

def deal_card ():

def deal_hand (ncards):

def shuffle():

def replace():

The way that the methods are implemented depends on the underlying representation. When the programmer calls deal(), they expect the method to return a card, which is an instance of the card class. How that happens is not relevant to them, but it is relevant to the person who implements the class. In addition, how it happens may be different on different computers, but as long as the result is the same, it does not matter.

For example, a card could be a constant value r that represented one of the 52 cards in the deck. The class could contain a set of values for these cards and provide them to programmers as a reference:

class card:

CLUBS_1 = 1

DIAMONDS_1 = 2

. . .

HEARTS_ACE = 51

SPADES_ACE = 52

Def __init__ (self, face, suit):

. . .

The variables CLUBS_1, DIAMONDS_1, and so on are accessible in all instances of the card class and have the appropriate value. Variables defined in this way have one instance only, and are shared by all instances.

A second implementation could be as a tuple. The ace of clubs would be (Clubs, 1), for instance. Each has advantages, but these will not be apparent to the user of the class. For example, the tuple implementation makes it easier to determine the suit of a card. This matters to games that have trump suits. The integer value implementation makes it easier to determine values and do specific comparisons. The value of a card could be stored in a tuple named ranks, for example, and ranks[r] would be a numerical value associated with the specific card.

Cat-a-Pult

Early in the development of personal computers, a simple game was created that involved shooting cannons. The player would set an angle and a power level and a cannonball would be fired towards the opposing cannon. If the ball struck the cannon, then it would be destroyed, but if not then the opposing player (or the computer) would fire back at the player’s cannon. This process would continue until one or the other cannon was destroyed. This game evolved with time, having more complex graphics, mountainous terrain, and more complex aspects. Its influence can be seen in modern games like Angry Birds.

FIGURE 6.1 An Example of the Cannon Game.

A variation of this game is proposed as an example of how classes can be used. The basic idea is to eliminate a mouse that is eating your garden by firing cats at it; hence the name cat-a-pult. The game will use text as input and output, because no graphics facility is available yet. A player types the angle and the power level, and the computer will fire a cat at the mouse. The location where the cat lands will be marked on a simple character display and the player can try again. The goal is to hit the mouse with as few tries as possible.

Basic Design

Before writing any code, one needs to consider the items in this game and the actions they can take. The items will be classes, the actions will be methods. There seem to be two items: a cannonball (a cat) and a cannon. The target (the mouse) could be a class too. The cannon has a location, an angle, and a power or force with which the cannonball will be ejected. Both of the last two factors affect the distance the ball travels. The cannon is given a target as a parameter—in this example the target will be another cannon, basically to avoid making yet another class definition.

The action a cannon can perform is to be fired. This involves releasing a cannonball with a particular speed and direction from the location of the cannon. In this implementation an instance of the cannonball class will be created when the cannon is fired and will be given the angle and velocity as initial parameters; the ball will, from then on, be independent. As a class, the ball has a position (x,y) and a speed (dx, dy). The action that it can perform is to move, which will be accomplished using a method named step(), and to collide with something, accomplished by the method testCollision().

Detailed Design

In the metaphor of this game, the cannonball is actually a cat and the target is a mouse, but to the program these details are not important. Here’s what is important:

Class Cannon

Class Ball

Has:

position x, y

position x, y

angle (when fired)

speed dx, dy

power (when fired)

name (text)

target (another cannon)

target (a Cannon class instance)

ball

gravity (force changing the height)

Does:

fire

step

step

test for collision

All of the Has aspects are class local variables, and in this design they will be initialized within the __init__ method of each class. This would entail the following:

self.x = x

self.x = x

self.y = y

self.y = y

self.power = 0

self.dx = dx

self.angle = 0

self.dy = dy

self.target = target

self.target = target

self.ball = None

self.gravity = 1.0

self.name = “”

The game is essentially one-dimensional. The cannonball will land at a specific x coordinate, and if that is near enough to the x coordinate of the target, then the target is destroyed and the game is over. Without a way to draw proper graphics, this can be imagined as a simple text display with the cannon on one side of the screen and the target on the other, something like that seen in Figure 6.1.

The slash character (“/”) on the left represents the cannon, and the “Y” represents the mouse, which is the target. The cannon is at horizontal coordinate 12, and the mouse is at 60; both vertical coordinates are 0.

All of the Does aspects represent actions, or things the class object can do. When the cannon is fired, the ball is created at the cannon coordinates (12, 0) and is given a speed that is related to the angle and power level using the usual trigonometric calculations learned in high school (Figure 6.2):

FIGURE 6.2 ASCII (text) video of the game at the beginning.

FIGURE 6.3 (Left) A review of how sines and cosines are computed.
(Right) using the definition of sine and cosine to calculate the speed
of the ball (or any object) in the x and y direction.

dy = sin(angle * 3.1415/180.0)

dx = cos(angle * 3.1415/180.0)

The angles passed to sin and cos must be in radians, so the value PI/180 is used to convert degrees into radians. The coordinates in this case have y increasing as the ball moves upwards. So, when the cannon is fired, a ball is created that has the x and y coordinates of the cannon and the dx and dy values determined as above. This is accomplished by a method named fire():

Fire: takes an angle and a power.

Angle is in degrees, between 0 and 360

Power is between 0 and 100 (a percentage)

1)compute values for dx and dy from angle and power, where max power is 0.1.

2)create an instance of Ball giving it x, y, dx, dy, a name (“cat”), and a target (the mouse).

The simulation makes time steps of a fixed duration and calculates positions of objects at the end of that step. Each object should have a method that updates the time by one interval, and it will be named step(). The cannon does not move, but sometimes it has a cannonball that it has fired, so updating the status of the cannon should update the status of the ball as well:

Step: make one time step for this object in the simulation. No parameter.

1)If a ball has been fired, then update its position. This is done by calling the step() method of the ball.

This defines the cannon.

The ball must also possess a step() method, and it will update the ball’s position based on its current speed and location. The x position is increased by dx, and the y is increased by dy. Gravity pulls down on the ball, effectively decreasing the vertical speed of the ball at each interval. After some trials it was determined that the value of dy should be decreased by the value of gravity at each interval. If the ball strikes the ground, it should stop moving. When does this happen? When y becomes smaller than 0. When this occurs, set dx and dy to 0 and check to see if the impact location is near to the target.

Step: make one time step for this object in the simulation. No parameter.

1)Let x = x + dx, changing the x position.

2)Let y = y + dy, changing the y position.

3)Decrease dy by gravity (dy = dy - gravity)

4)if the ball has struck the ground

5)let dx = dy = gravity = 0

6)check for collision with target

Checking to see if the ball hit the target is a matter of looking at the x value of the ball and the x value of the target. If the difference is smaller than some predefined value, say 1.0, then the target was hit. This is determined by a method that will be called testCollision(). If the collision occurred then success has been achieved by the player, so set a flag that will end the game.

testCollision: check to see if the ball has hit the target; if so, set a flag.

1)subtract the x position of the ball from the x position of the target. Call this d.

2)if d <= 1.0 then set a flag done to True.

This defines the class Ball and completes the two major classes.

The main program that uses these classes could look something like this:

mouse = Cannon (60, 0, None)        # Create the target

player = Cannon (12, 0, mouse)        # create the cannon

player.fire (42, 65)        # Example: fire cannon at 42 degrees 65% power

done = False         # initialize variable 'done'

while not done:        # so long as the simulation is not over

player.step()        # Update the position of the ball.

The previous process loosely defines a way to design and code a program that uses classes.

FIGURE 6.4 The Ball and the Cannon classes from the Cat-a-pult simulation.

FIGURE 6.5 Frames from the text animation of the game.

Subclasses and Inheritance

Classes are designed as language features that can represent a hierarchy of information or structure. A class can be used to define another, and properties from the first class will be passed on (inherited) by the other. A class that is based on another in this way is called a subclass, and explanatory examples suffuse the Internet: a pet class with dogs and cats as special cases; a polygon having triangles and rectangles as subclasses; a dessert class, having subclasses pie, cake, and cookie; even the initial example in this chapter of the man class and a hypothetical person class that it could be derived from. A subclass is a more specific case of the superclass (or parent class) on which it is based.

The examples above are for explanation, and are not really useful as software components, which begs a question about whether subclasses are really useful things. They are, but it requires non-trivial examples to really demonstrate this.

Non-Trivial Example: Objects in a Video Game

To some degree all objects in a game have some things in common. They are things that can interact with other game objects; they have a position within the volume of space defined by the game and they have a visual appearance. Thus, a description of a class that could implement a game object would include:

class gobject:

position = (0, 0, 0)        # Object position in 3D

visual = None        # Graphics that represent the object

def __init__ (self, pos, vis)

def getPosition (self):

def setPosition(self, p):

def setVisual(self, v):

def draw (self):

Anyone who has played a video game knows that some of the objects can move while others cannot. Objects that move can have their position change, and it has to be updated regularly. An object that can move can have a speed and a method that updates its position; otherwise it is like a gobject. This is a good case for a subclass:

class mobject (gobject):

speed = (0, 0, 0)        # Speed in pixels per frame the

# x,y,z directions

def __init__ (self, s)

def getSpeed(self):

def setSpeed(self, s):

def move(self):

def collision(self, gobject):

The syntax of this has the superclass gobject as a parameter (apparently) of the subclass mobject being defined. If an instance of a gobject is created, its __init__ method is called and the resulting reference has access to all of the methods in the gobject definition, just as one would expect. If an instance of mobject is created, the __init__ method of mobject is called, but not that of gobject. Nonetheless, all properties and methods of both classes are available through the mobject reference; that is, the following is legal:

m = mobject ( (12, 0, 0))        # Create mboject with speed (12,0,0)

m.draw()         # Draw this object

even though an mobject does not possess a method draw(); the method defined in the parent class is accessible and will be used. When the mobject is created, it is also a gobject, and all of the variables and methods belonging to a gobject are defined also. However, the __init__() method for gobject is not called unless the mobject __init__() method does so. This means that, for the mobject, the values of position and visual are not specified by the constructor and will take the default values they were given in the gobject class. If no such value was given, they will be undefined and an error will occur if they are referenced.

Calling the __init__() method of the parent class can be done as follows:

super().__init__((10,10,10), None)

In this instance the constructor for gobject is called, passing a position and a visual. This would normally be done only in the __init__() of the subclass.

Now consider the following code. The methods are mainly stubs that print a message, but the output of the program is instructive:

class gobject:

# Object position in 3D

# position = (0, 0, 0)

# Graphics that

# represent the object

visual = None

def __init__

(self,pos,vis):

self.position = pos

self.visual = vis

print ("gobject init")

def getPosition(self):

return self.position

print

("getPosition")

def setPosition(self,

p):

self.position = p

print

("setPosition")

def setVisual(self,

v):

self.visual = v

print ("setVisual")

def draw (self):

print("Draw")

class mobject (gobject):

# Speed in pixels per frame

# the x,y,z directions

speed = (0, 0, 0)

def __init__ (self, s):

self.speed = s

super().__init__

((10,10,10), None)

print ("mobject init")

def getSpeed(self):

print ("getSpeed")

return self.speed

def setSpeed(self, s):

print ("setSpeed")

self.speed = s

def move(self):

print ("Move")

def collision(self,

gobject):

print ("collision")

g = gobject ((12, 12,12), None)

m = mobject((13,13,13))

print (m.getPosition())

m.move()

m.draw()

Output from this is:

gobject init

from the creation of the gobject instance g

gobject init

when m is created it calls the parent __init__

mobject init

from the mobject __init__ when m is created

(10, 10, 10)

m.getPosition, showing access to parent methods

Move

m.move call

Draw

m.draw call, again showing access to parent method

Attempting to call g.move() would fail because there is no move() method within the gobject class. Hence if an object was passed to a function that would attempt to move it, it would be critical to know whether the parameter passed was a gobject or an mobject. Consider a method that moves an object x out of the path of an mobject instance if it can, or changes the path of the mobject if it cannot. This method, named dodge(), might do the following:

def dodge self, (x):

c = x.getPosition()

c = c + (dx, dy, 0)

x.setPosition (c)

However, if the parameter is an instance of a gobject, then it should not be moved. The function isinstance() can be used to determine this. The result of:

isinstance (x, gobject)

will be True if x is a gobject and False otherwise. If False then it can’t be moved and the dodge() method will have to move the current mobject out of the way instead:

def dodge self, (x):

if isinstance(x, gobject):

self.position = self.position + (dx, dy, 0)

else:

c = x.getPosition()

c = c + (dx, dy, 0)

x.setPosition (c)

Duck Typing

In many programming languages types are immutable and compatibility is enforced. This is not generally true in Python, but still there are operations that require specific types. Indexing into a string or tuple must be done using something much like an integer, and not by using a float. Now that classes can be used to build what amounts to new types, more attention should be paid to the things a type should offer and the requirements this puts on a programmer. A Python philosophy could be that the fewer restrictions the better, and this is a principle of duck typing as well.

It should not really matter what the exact type of the object is that is being manipulated, only that it possesses the properties that are needed. In a very simple case, consider the classes point and triangle that were discussed at the beginning of this chapter. It was proposed that both could have a draw() method that would create a graphical representation of these on the screen, and both have a move() method as well. A function could be written that would move a triangle away from a point and draw them both:

def moveaway (a, b)

dx = a.getx()-b.getx()

dy = a.gety()-d.gety()

a.move (dx/10, dy/10)

b.move (-dx/10, dy/10)

Question: which of the parameters, a or b, is the triangle, and which is the point? Answer: it does not matter. Both classes have the methods needed by this function, namely getx(), gety(), and move(). Because of this the calls are symmetrical, and both of the following are the same:

moveaway (a, b)

moveaway (b, a)

In fact, any class that possesses these three methods can be passed to moveaway() and a result will be calculated without error. The essence of duck typing is that, so long as an object offers the service needed (i.e., a method of the correct name and parameter set) to another function or method, then the call is acceptable. There is a way to tell whether the class instance a has a getx() method. The built-in function hasattr():

if hasattr (v1, "getx"):

x = v1.getx()

The first argument is a class instance and the second is the name of the method that is needed, as a string. It returns True if the method exists.

The name comes from the old saying that “if something walks like a duck and quacks like a duck, then it is a duck.” As long as a class offers the things asked for, then it can be used in that context.

CHAPTER 7

GRAPHICS, MEDIA, AND INTERFACES

At the most primitive level of graphics software is the ability to set individual pixels. It is quite difficult to use this capability to create complex pictures. How is a dog drawn, or a building, or even just a straight line? Fortunately, those things have been figured out.

At the bottom layer of software are functions that manipulate pixels. At the next level are lines and curves; these are the basic components of drawings and sketches. An artist with a pencil uses lines and curves to represent scenes. At the level above lines are functions that use lines to create other objects, such as rectangles, circles, and ellipses. These can be line drawings or can be filled with colors. The next higher levels can be argued about, but text is probably in the next software layer and then shading and images followed by 3D objects, which includes perspective transformation and textures.

Python itself does not have graphics tools, but various modules that are associated with Python do. The standard graphical user interface library for use with Python is tkinter. There are many features of this module, including the creation of windows, drawing, user interface widgets such as buttons, and a host of other features. It is free and is normally included in the Python distribution.

Another library that allows graphics programming is called Pygame, and this is designed for building computer games using Python. Let’s look in detail at Pygame, as it will allow us to draw pictures, manage interfaces, and do animations.

Installing Pygame

Download Pygame for Windows from:

http://pygame.org/ftp/pygame-1.9.1.win32-py2.7.msi

Apple:

http://pygame.org/ftp/pygame-1.9.1release-py2.6-macosx10.5.zip

Ubuntu Linux users can type sudo apt-get install python-pygame. Other Linux and Unix distributions can be downloaded through http://www.pygame.org/download.shtml

And follow the installation notes at:

http://www.pygame.org/wiki/GettingStarted

Essentials: The Graphics Window and Colors

To start creating computer graphics, it is necessary to understand how Pygame manages the screen and other resources. There is a distinct set of steps that must be followed in order for even the simplest Pygame program to work. After the basic steps are accomplished, we can draw into a graphics window and have it appear on the screen.

The first step is to import the Pygame library. Assuming that it has been installed correctly, this is a matter of beginning with the statement:

import pygame

FIGURE 7.1 The Pygame Window.

Next, there are variables that need to be initialized and storage that has to be allocated for Pygame to work. One example is that fonts must be loaded and placed into a data structure. This is done with the statement:

pygame.init()

Nothing seems to happen, but Pygame is now ready to work. Next we create a drawing structure called a surface:

surf = pygame.display.set_mode((400, 450))

This surface will be 400 pixels wide by 450 pixels high, and will appear briefly on the screen and will then vanish. Why vanish? Because the program ends after the last statement taking the window with it.

The variable surf will contain a reference to a surface object, and in this case it will be the display surface, because we accessed it through the display part of the pygame object. The display surface is the place where things are drawn if we want them to be visible on the screen. There are other surfaces that can be drawn on that will not display by default. The method set_mode takes a tuple as a parameter that gives the size of the surface.

How can we keep the drawing area on the screen? Don’t end the program until told! We could, as one example, read something from the keyboard and then terminate the program. Here’s the first full Pygame program, which is nonstandard but functional:

import pygame

pygame.init()

surf = pygame.display.set_mode((400, 400))

pygame.display.update()

input()

The window will stay on the screen until a character is typed in the input region (not the drawing window!)

Simple Static Drawing

Everything drawn on the display surface has a color, and it is a tuple consisting of the red, green, and blue component of the color. Thus, the tuple (255,255,255) can be a color, and would be white. (0,0,0) would be black. To humans, colors have names. Here’s a list of some named colors and their RGB equivalents:

The background is black by default. Assuming that the display surface is named surf, then the background color can be changed by a call to the fill method, passing a tuple specifying the color:

surf.fill ((255, 0, 0))

In this case the background color will be red. Pygame also has a Color class that has red, green, and blue components and methods for converting to non-RGB color specifications like HSV. After:

c = pygame.Color(255,0,0)

surf.fill (c)

the color stored in c will be red as will the background color.

Pixel-Level Graphics

The only pixel-level operation draws a pixel at a specified location; so, for example, the call:

surf.set_at ((x, y), c)

will set the pixel at coordinates (x,y) to the color c. Setting a collection of pixels that are adjacent to each other will create a line.

Example: Create a Page of Notepaper

Notepaper has blue lines separated by enough space to write or print text between them. It often has a red vertical line indicating an indentation level, a place to begin writing. Drawing this is a matter of drawing a set of connected blue pixels in vertically separated rows, and then making a vertical column of red pixels. Here is one way to code this:

FIGURE 7.2 A graphic of a sheet of lined paper.

import pygame

pygame.init()

surf = pygame.display.set_mode((400, 400))

c = pygame.Color(0,0,200)

surf.fill ((255,255,255))

y = 60         # Height at which to start

for n in range (0, 27):         # Draw 30 horizontal blue lines

for x in range (0,400):        # Draw all pixels in one line

surf.set_at ((x, y),c)        # Draw a blue pixel

y = y + 20                 # The next line is 20 pixels down

c = (200, 0, 0)                 # Pixel color red

for y in range (0, 400):                # Draw connected vertical pixels

surf.set_at ((25, y), c)        # to form the margin line

pygame.display.update()

input()

The output of this program is shown in Figure 7.2. When pixels are drawn immediately next to each other they appear to be connected, and so in this case they form horizontal and vertical lines. This is not easy to do for arbitrary lines; it is not obvious exactly which pixels to fill for a line between, say, (10, 20) and (99, 17). That’s why the line-drawing functions exist.

Example: Creating a Color Gradient

When creating a visual on a computer, the first step is to have a clear picture of what it will look like. For this example, imagine the sky on a clear day. The horizon shows a lighter blue than the sky directly above, and the color changes continuously all the way from horizon to zenith. If a realistic sky background were needed, then it would be necessary to draw this using the tools available. What would the method be?

First, decide on what the color is at the horizon (y=ymax) and at the highest point in the scene (y=ymin). Now ask: “how many pixels between those points?” The change in pixel color will be the color difference from ymax to ymin divided by the number of pixels. Now simply draw rows of pixels beginning with the horizon and moving up the image (i.e., decreasing Y value), changing the color by this amount each time.

As an implementation, assume that the color at the horizon will be blue = (40, 40, 255) and the top of the image will be (40, 40, 128), a darker blue. The height of the image will be 400 pixels; the change in blue over that range is 127 units. Thus, the color change over each pixel is going to be 255.0/400. A color can’t change a fractional amount, of course, but what this means is that the blue value will decrease by approximately 1 unit for the increase in height of every couple of pixels. Do not forget that the horizon is at the bottom of the image, which has the greatest Y coordinate value, so that an increase in Y means a decrease in height and vice versa.

The example program that implements this is:

import pygame

pygame.init()

surf = pygame.display.set_mode((400, 400))

surf.fill ((255,255,255))

blue = 0

delta = 255.0/400

for y in range (0, 400):

yy = 400-y

c = (40, 40, blue)

for x in range(0, 400):

surf.set_at ((x, y), c)

blue = blue + delta

pygame.display.update()

input()

Figure 7.3 shows what the gradient image looks like in greys.

FIGURE 7.3. A color gradient drawn as pixels.

Lines and Curves

Straight lines and curves are more complex objects than pixels, consisting of many pixels in an organized arrangement. A line is actually drawn by setting pixels, though. The fact that a line() function exists means that the programmer does not have to figure out what pixels to draw and can focus on the higher level construct, the line or curve.

A line is drawn by specifying the endpoints of the line. Using Pygame the call is:

pygame.draw.line ( surf, col, (x0, y0), (x1, y1))

where one end of the line is at (x0,y0) and the other is at (x1,y1). The color of the line is specified by the second parameter col. If any part of the line extends past the boundary of the window that’s OK; the line will be clipped to fit.

Example: Notepaper Again

The example of drawing a piece of notepaper can be done using lines instead of pixels, and will be a lot faster. Draw a collection of horizontal lines (i.e., that have the same Y coordinate at the endpoints) separated by 20 pixels, as before having a blue color. Then draw a vertical red line for the margin. The program is a variation on the previous version:

import pygame

pygame.init()

y = 60         # Height at which to start

width = 400

height = 400

surf = pygame.display.set_mode((width, height),pygame.SRCALPHA)

surf.fill ((255,255,255))

y = 60         # Height at which to start

for n in range (0, 27):        # Draw 30 horizontal blue lines

pygame.draw.line (surf, (0,0,200), (0, y), (width, y))

y = y + 20         # The next line is 20 pixels down

c = (200, 0, 0)         # Pixel color red

pygame.draw.line (surf, c, (25,0), (25,height))

pygame.display.update()

input()

The output from this program is the same as that for the version that drew pixels, which is shown in Figure 7.2.

A curve is trickier than a line, in that it is harder to specify. The method used in Pygame is common: a curve (arc) is defined as a portion of an ellipse from a starting angle for a specified number of degrees, as referenced from the center of the ellipse. Here’s a call to arc:

pygame.draw.arc (surf, c, box, start_angle, end_angle)

The parameter surf is the surface to draw on, c is the color, box is an enclosing bounding box as a tuple (upper left x, upper left Y, width, height), start _angle is an angle between 0 and 2π radians, and stop_angle is an angle in the same range. The angle 0 is to the right, 90 degrees is up, 180 degree (π radians) is left, and 270 degrees is down. The angle specifies the part of the ellipse to draw. So:

FIGURE 7.4 ARC example 1.

FIGURE 7.5 ARC example 2.

FIGURE 7.6 ARC example 3.

The curves are drawn counterclockwise. The value conv is π/180, and converts an angle in degrees into radians when multiplied.

Polygons

For the purposes of discussion, a polygon will include all closed regions, including ellipses and circles.

FIGURE 7.7 Filled rectangle.

A rectangle is drawn using the rect method.

Pygame.draw.rect (surf, ((0,200, 50), (100, 100, 200, 300))

The surf and color parameters are as before, and the box is specified as the upper left coordinates, the width, and the height. By default the rectangle is filled with the specified color.

FIGURE 7.8 Unfilled rectangle.

An additional final argument specifies the line thickness with which to draw the rectangle, and if this is specified then the rectangle is not filled with color:

FIGURE 7.9 Unfilled ellipse.

pygame.draw.rect (surf, (0,200, 50), (100, 100, 200, 100), 1)

The ellipse method takes the same parameters as does rect, and draws an ellipse within the rectangle defined by the third parameter.

pygame.draw.rect (surf, (230,230, 0), (100, 100, 200, 100), 1)

pygame.draw.ellipse (surf, (0,200, 50), (100, 100, 200, 100), 1)

A circle is an ellipse drawn in a square. This makes the center and radius rather implicit. There is a circle method also:

pygame.draw.rect (surf, (230,230, 0), (50, 50, 100, 100), 1)

pygame.draw.circle (surf, (0,200, 50), (100, 100), 50)

FIGURE 7.10. Filled circle.

The third parameter to a circle is a tuple defining the center, and the fourth is the radius. A fifth would be the line thickness, and filling would turn off. In the case here of a circle at (100,100) and a radius of 50, the enclosing square would be from (100-50, 100-50), which is (50, 50), for (100,100) pixels.

Blitting

To blit is to combine several graphics or bitmaps into a single one. It is often accomplished using a Boolean function, and often is very fast due to hardware assistance. Pygame has one special Surface that is the display Surface, but allows us to draw on other surfaces too. To display what is drawn on these surfaces we would blit them to the display Surface.

Blitting has consequences and requires specifications that are not usually appreciated by the definition. Consider the creation of two Surfaces named s1 and s2 in addition to the display surface, and drawing into each of those:

s1 = pygame.Surface((400,400))        # New Surface

pygame.draw.rect (s1, (230,230, 0), (50, 50, 100, 100), 1)

s2 = pygame.Surface((400,400))        # New Surface

pygame.draw.circle (s2, (0,200, 50), (100, 100), 50)

The Surface s1 contains a rectangle, and the Surface s2 contains a circle. Neither appears on the display Surface, which as usual is named surf. A blit is a copy from one Surface to another. Some questions are:

Which part of the Surface being blitted is copied?

Where (coordinates) is the surface being blitted to?

What happens to the pixels that already exist in the region being blitted to?

The method that copies (blits) one surface to another is blit, the simplest form of which is:

surf.blit (s1, (0,0))

This copies all of Surface s1 to surf so that the upper left of s1 is at (0,0) of surf. We can copy s1 to any pixel coordinate in surf. To draw a circle and a rectangle in different Surfaces and then blit them to the display Surface would involve creating the surfaces, drawing in them, and blitting them:

s1 = pygame.Surface((200,200))        # S1 is 200x200

s1.fill ((255,255,255))         # White background

pygame.draw.rect (s1, (230,230, 0), (50, 50, 100, 100), 1)

s2 = pygame.Surface((200,200))        # s2 is also 200x200 pixels

s2.fill ((255,255,255))         # White background too

pygame.draw.circle (s2, (0,200, 50), (60, 60), 50)

# Blit rectangle to (0,0) and circle to (100,100)

surf.blit (s1, (0,0))         # s1 has a rectangle: blit

surf.blit (s2, (100,100))         # s2 has a circle: blit

pygame.display.update()

Here s1 is blitted before s2 (i.e., is drawn first), and there is overlap between the drawn regions. Thus, the one drawn last (s2) appears to be drawn over s1. If we think in terms of layers, the last surface drawn is the top layer, and is visible. Layers beneath may be partly or completely covered by layers above. A Surface is rectangular, so notice that the background surrounding the circle is also drawn over the square below.

The blit function has other parameters that we’ll get into shortly.

FIGURE 7.11 One surface blitted to another.

Drawing Text

Drawing text is accomplished by loading a font and then drawing (rendering) a text string to a surface using that font as a guide. An instance of the Font class, and there is a default for that, can render text onto a surface. That surface is then blitted to the target surface, possibly the display. A simple example involves placing the text “Hello there” at location (100,100):

pygame.init()         # Must initialize pygame and fonts

font = pygame.font.Font(None, 36)

text = font.render("Hello There", 1, (10, 10, 10))

surf.blit (text, (100,100))

The method pygame.font.Font selects a font to be used and returns an instance. A font has a name, in this case None indicating that we should use the default, and a size, in this case 36. Each computer system has a different set of fonts available, so we’ll use the default. Next, the font class can draw (render) the text onto a surface. The call:

text = font.render("Hello There", 1, (0,0,255))

renders the text “Hello there” in the color (0,0,255), which is blue. The second parameter 1 means to anti-alias, which will yield nice smooth characters. Finally:

surf.blit (text, (100,100))

will blit the text to the display Surface surf at location (100,100). The coordinates (100,100) are those of the upper left of the text Surface, which will be a rectangle large enough to enclose the string.

FIGURE 7.12 Rendering text.

A problem is that this text Surface will write over anything underneath as a rectangular area. This can be fixed by using a transparent background.

Transparent Colors

When one pixel is drawn over the top of (i.e., at the same location as) another, the one drawn most recently will be visible. This may not always be what is needed. Background pixels of text images being blitted should be invisible so that the background can be seen with the text on top.

Transparency is a value that can be numerical. Let’s say that a value of 0 means that the drawn pixel is invisible and a value of 255 means that it is opaque. Values in between have degrees of transparency. Then we want the background of a text box to have a value of this parameter of 0, and the text to have a value of 255. Looking at this value it has the same properties as does a colour component, and so it is generally implemented as a fourth component called alpha. A color can be specified as RGBA, which means four components: red, green, blue, and alpha.

Not all Surface objects can implement transparency. They must have a property called 32-bit color and have the SCRALPHA property. Creating a Surface like this is done as follows:

surf = pygame.display.set_mode((w, h), pygame.SRCALPHA, 32)

where the third parameter means that the Surface can support transparency and the final one means that it has 32-bit colors: 4 values of 8 bits each.

FIGURE 7.13 Surface with transparent background (circle) blitted to another.

The previous example having a rectangle and a circle drawn and then blitted to the display Surface can now be implemented using transparency:

surf = pygame.display.set_mode((400, 400), pygame.SRCALPHA)

surf.fill ((255,255,255))

s1 = pygame.Surface((200,200), pygame.SRCALPHA, 32)

s1 = s1.convert_alpha()

s1.fill ((255,255,255, 0))

pygame.draw.rect (s1, (230,230, 0), (50, 50, 100, 100), 1)

s2 = pygame.Surface((200,200), pygame.SRCALPHA, 32)

s2 = s2.convert_alpha()

s2.fill ((255,255,255, 0))

pygame.draw.circle (s2, (0,200, 50), (60, 60), 50)

surf.blit (s1, (0,0))

surf.blit (s2, (100,100))

pygame.display.update()

input()

The fill color value of (255,255,255,0) yields a fully transparent color that will comprise the background of the circle and the rectangle Surface, allowing the background to show through.

Images

Unlike the graphical components displayed so far, an image is fundamentally a collection of pixels. A camera captures an image and stores it digitally as pixels, and so it was never anything else. Displaying an image means drawing each pixel in the appropriate color, as captured. Pygame can load and display images in files of various formats: JPEG, GIF, BMP, and PNG.

FIGURE 7.14 The original “Checkpoint Charlie” image.

Unlike languages such a Java, Python has no image class. An image is read from a file using the function pygame.image.load and is returned as a Surface. This means that it can be displayed immediately using a blit and that individual pixels can be accessed using the Surface method get_at().

The file “charlie.gif” is a photo of checkpoint Charlie in Berlin (Figure 7.14). It could be read in to a Python program with the call:

im = pygame.image.load ("charlie.gif")

The variable im now holds the image and can be displayed using:

surf.blit (im, (0,0))

While the details are not completely relevant, it is good to know that im.get_width() and im.get_height() give the width and height of the image in pixels.

The complete Python program (using Pygame) that can load and display the image is thus:

import pygame

pygame.init()

im = pygame.image.load ("charlie.gif")

width = im.get_width()

height = im.get_height()

surf = pygame.display.set_mode((width, height),

pygame.SRCALPHA)

surf.fill ((255,255,255))

surf.blit (im, (0,0))

pygame.display.update()

input()

This displays the image in a window that is exactly the correct size.

Pixels

An image, as has been mentioned, is just a Surface after being read. Individual pixels can be accessed using the method get_at passing the x and y coordinates. The code

pix = im.get_at ((i,j))         # Parameter is a tuple

returns the color of the pixels at (i,j), which is a tuple containing red, green, blue, and alpha components.

Setting the value of the pixel at location (x,y) is accomplished by calling im.set_at().

im.set_at ((x,y), color)

where again, color is a tuple.

Example: Thresholding

Thresholding is an early step in many image analysis processes. It is the creation of a bi-level image, having just black and white pixels, from a grey or color image. In thresholding a simple grey value T, the threshold, is used to separate pixels into black and white: all pixels having a value smaller that T will be black, and the others will be white.

FIGURE 7.15 Thresholded “Checkpoint Charlie” image.

Using the Checkpoint Charlie image again, let’s create a thresholded image using a threshold of 128, the middle of the possible range. First we have to convert the color pixels into grey values. An easy way to do this is to average the R, G, and B values. For a pixel value pix:

grey = (pix[0]+pix[1]+pix[2])/3

Now if this value is smaller than the threshold 128, the corresponding pixel in the image is set to black, or (0,0,0); otherwise it is set to white, or (255,255,255). A program that does this is:

import pygame

pygame.init()

im = pygame.image.load ("charlie.gif")

width = im.get_width()

height = im.get_height()

for i in range (0,width):

for j in range(0,height):

pix = im.get_at ((i,j))

grey = (pix[0]+pix[1]+pix[2])/3

if grey < 128:

im.set_at ((i,j), (0,0,0))

else:

im.set_at ((i,j), (255,255,255))

surf = pygame.display.set_mode((width, height), pygame.SRCALPHA)

surf.blit (im, (0,0))

pygame.display.update()

input()

Interaction

The code written so far generates and displays a single image and then waits for the user to dismiss it with a keystroke. This is a simplification of how Pygame usually works. Because Pygame was developed so that computer games could be written in Python, it must permit animation, which means it can display a new image or frame every fraction of a second. Consider a ball represented by a circle at the location (10,10) on a surface. If the ball is supposed to be moving, then the coordinates will change and it will be drawn again, this time at the new position, say (10,12). It will be drawn again at (10,14), and so on until it disappears off of the bottom of the display. This is how animation works: by the rapid display of images that incorporate the frame-by-frame changes because of motion.

But how do we display a sequence of images rather than just a single one? The call to input() at the end of the program prevents it from terminating, and allows the final image to stay on the screen. To display an animation we would need to display an image, wait for a fraction of a second, and then display another one, over and over. This is actually how Pygame was intended to be used.

In principle a loop could be used to do this. Instead of the call to input(), code a loop that includes changes to the graphic rendered in the Surface and then update the display surface. Here’s an example, which will draw a blue circle that moves from the left to the right side of the screen:

import pygame

pygame.init()

x = 0

y = 100

surf = pygame.display.set_mode((400, 400), pygame.SRCALPHA)

while True:

surf.fill ((255,255,255))

pygame.draw.circle (surf, (0,0,200), (x,y), 20)

x = x + 1

pygame.display.update()

This is an infinite loop, and this particular one is not a recommended solution. Each time through this loop, the position of the circle is changed and the display Surface is updated, which is to say it is drawn to the screen. There is no gentle way to terminate the program, and the speed is hard to manage as well. There is a standard Pygame main loop used by most games that shows how the Surface is to be managed. It consists of an infinite loop that can control the time between iterations and can deal with user interactions between frames.

Time

A typical 32-mm film displays a new image 24 times per second. A television does so 30 times per second. Pygame allows the control of duration using a time module and the Clock class. Create an instance:

clock = pygame.time.Clock()

This class has a tick method. The call:

t = tick()

will return the number of milliseconds that have passed since the last time tick was called. Better yet, the call:

tick(30)

will not return until 1/30 of a second has passed since the last time tick was called. This allows us to run the loop at a standard rate:

clock = pygame.time.Clock()

while True:

clock.tick(30)         # Frame rate is 30 per second

surf.fill ((255,255,255))

pygame.draw.circle (surf, (0,0,200), (x,y), 20)

x = x + 1

pygame.display.update()

Running this program shows the blue circle moving at a more sedate speed across the screen, moving 30 pixels each second.

Events

The best definition for an event is “something that happened.” Some events can be predicted, such as when the blue circle reaches the right side of the screen. That time can be computed. Other events cannot: a key press by the user can’t be predicted, nor can a mouse gesture. When using clock.tick(30) to control the speed of the program, it’s possible that a user could press a key or two or move the mouse between interactions of the loop. Pygame keeps track of these events and remembers them for use, and they can be dealt with at the beginning of each loop.

The function pygame.event.get() will return a representation of a Pygame event that happened; in fact it will be the oldest one that has not been looked at. There may be more than one event that has not been looked at, so the function should be called for all recalled events:

for event in pygame.event.get():

Each time through this loop, the variable event will be an event that needs to be dealt with. Events can be:

QUIT

Program complete

KEYDOWN

Key was pressed

KEYUP

Key was released

MOUSEMOTION

The mouse has been moved

MOUSEBUTTONUP

A mouse button is released

MOUSEBUTTONDOWN

A mouse button is pressed

There are others, but these are the big six. Let’s look at these events as they will actually be used.

The Mouse

The mouse is used to identify components on the display surface that the user would like to manipulate in some way. In addition to the three mouse events listed above, the mouse has a location, which is the position of the mouse cursor in the display window. The method mouse.get_pos() will return a tuple that has the x and y components of the mouse position. For example, the following program will draw a blue circle at the current mouse position:

import pygame

pygame.init()

clock = pygame.time.Clock()

surf = pygame.display.set_mode((400, 400), pygame.SRCALPHA)

while True:

clock.tick(30)

for event in pygame.event.get():

pass

surf.fill ((255,255,255))

pygame.draw.circle (surf, (0,0,200),

pygame.mouse.get_pos(), 20)

pygame.display.update()

Here the position of the circle, a tuple at the third parameter of the circle call, is the position of the mouse. This works fine. The check for events seems to not do anything here, but must be present for the program to work properly; otherwise the circle is drawn at one position and nothing else happens.

A mouse click or button press is an event, and can be identified in the for loop. The main issue when a mouse button press or release is determined is the location of the mouse when that happens. A release when the cursor is inside of a specific rectangle could be a button press, for example.

Button

Consider a rectangle at location (100,100) that is 100x30 pixels in size. When the mouse button is released while inside of this region, something will happen—the background color of the display Surface will change. This represents a button in the general sense of a computer user interface, and works in the same way.

FIGURE 7.16 Button example; changing background color.

The program begins as usual, but the for loop has something in it: it tests the mouse position when a button-release event is detected, and if that position lies within the selected area, the background color is changed by modifying the red component, toggling it between 0 and 255.

import pygame

pygame.init()

clock = pygame.time.Clock()

red = 255

surf = pygame.display.set_mode((400, 400), pygame.SRCALPHA)

while True:

clock.tick(30)

for event in pygame.event.get():

if event.type == pygame.MOUSEBUTTONUP:

p = pygame.mouse.get_pos()

if p[0]>100 and p[0]<200

and p[1]>100 and p[1]<130:

red = 255-red

surf.fill ((red,255,255))

pygame.draw.rect (surf, (0,0,200), (100,100,200,30))

pygame.display.update()

Stretchy Lines

The problem is this: draw a line between two points. The first click of the mouse defines the starting point of the line, and the second click defines the end. The second point will follow the mouse until the second click occurs. The event managing the loop now looks for two events. When the MOUSEBUTTONDOWN event happens, the mouse position is used to define the point (x0,y0). When MOUSEBUTTONUP occurs then the position (x1,y1) is defined. A line is drawn between these two points. If x0 = 1 then it has not been defined, and nothing is drawn. If x1 = 1 then a line is drawn between (x0,y0) and the current mouse position.

FIGURE 7.17 Draw a line using the mouse.

Import pygame

pygame.init()

clock = pygame.time.Clock()

x0,y0 = -1,-1

x1,y1 = -1,-1

surf = pygame.display.set_mode((400, 400), pygame.SRCALPHA)

while True:

clock.tick(30)

p = pygame.mouse.get_pos()

for event in pygame.event.get():

if event.type == pygame.MOUSEBUTTONUP:

x1,y1 = p[0],p[1]

if event.type == pygame.MOUSEBUTTONDOWN:

x0,y0 = p[0],p[1]

surf.fill ((255,255,255))

if x1 >= 0:

pygame.draw.line (surf, (0,0,200), (x0,y0),(x1,y1))

elif x0 >= 0:

pygame.draw.line (surf, (0,0,200), (x0,y0),(p[0],p[1]))

pygame.display.update()

The Keyboard

Keyboard events are limited to KEYDOWN and KEYUP, but there are a variety of keys that might have been pressed. When a keyboard event occurs, there is an accompanying variable key that is part of the event that indicates what key is involved. Keys all have special named constants: the letter “A” is KEY_A, for example. The critical keys for games and interaction would be the arrow keys K_LEFT, K_RIGHT, K_UP, and K_DOWN, the equivalent keys as characters K_w, K_a, K_s, and K_d, and the space key K_SPACE. These are frequently used in games. A list of characters is available at http://www.pygame.org/docs/ref/key.html.

An event for loop that would detect the up arrow key being pressed would be:

for event in pygame.event.get():

if event.type == pygame.KEYDOWN:

if event.key == pygame.K_UP:

print ("Up arrow")

pygame.display.update()

Sound

Sound is complicated in Pygame. Pygame has a mixer class that provides much of the functionality, allowing files to be read in, multiple channels to be mixed together and played, volume control, and so on. It would be a whole chapter, so the basics are as follows. When a sound file is to be played:

1)Import pygame.mixer and initialize it.

import pygame.mixer

pygame.mixer.init()

2)Read the sound file into a variable.

sound = pygame.mixer.Sound ("sound.wav")

3)Play the sound.

sound.play()

Other aspects like mixing will not be discussed. A program that will do this is:

import pygame

import pygame.mixer

pygame.init()

pygame.mixer.init()

clock = pygame.time.Clock()

sound = pygame.mixer.Sound ("sound.wav")

sound.play()

surf = pygame.display.set_mode((400, 400), pygame.SRCALPHA)

surf.fill ((255,255,255))

while True:

clock.tick(30)

. . .

A sound file has to be in WAV format, but this may change in later versions of Pygame. Pygame can also play video files. See http://www.pygame.org/docs/ref/movie.html for details.

A Game

Because Pygame was designed to help create games, it only seems reasonable to use a game as a final example. It will have to be simple to fit in the available space, and it should use as much of the functionality that we have learned as is possible. Such a simple game is Lunar Lander, a 1970s arcade game that has the player attempting to land a spacecraft on the moon.

FIGURE 7.18 The Lunar Lander game.

The game begins with the craft near the top of the screen. It immediately begins to fall to the surface, which is near the bottom of the screen. Pressing the UP key powers the engines and slows the craft’s fall, but also uses fuel. LEFT and RIGHT keys cause the craft to move left and right, and also use fuel. If the craft touches the ground slowly enough, then a successful landing occurs. Otherwise not.

The controls are UP, LEFT, and RIGHT. The object in the game being controlled is a lunar lander, and the background is a lunar landscape, seen in profile. The small number of objects make the game easier to implement than some others.

The Lander

The lander graphic is intended to be a low-resolution rendering of the NASA lander. The graphic origin of this drawing is the upper left corner of the grey rectangle representing the body. There is a circle above and landing legs below. The two small extensions to the landing legs are not drawn in the game but represent landing sensors. When they touch the ground the craft has landed, or it has at least touched the ground.

The lander will be drawn by a function that takes as the (x,y) parameters the location of the origin in the drawing area.

FIGURE 7.19 The Lander graphic.

def draw_lander (x, y):

global power

pygame.draw.rect (surf,(200,200,200),(x, y,20, 20))

pygame.draw.circle(surf,(250,250,90),(x+10,y-10), 10)

pygame.draw.line (surf,(200,200,200),

(x+5,y+20), (x+3,y+22))

pygame.draw.line (surf,(200,200,200),

(x+3,y+22), (x+18,y+22))

pygame.draw.line (surf,(200,200,200),

(x+18,y+22), (x+15,y+20))

pygame.draw.line (surf,(200,200,200),

(x+10,y+10), (x-5, y+25))

pygame.draw.line (surf,(200,200,200),

(x+10,y+10), (x+25, y+25))

The lander will be drawn at a location on the display window indicated by a pair of global variables x and y. The speed at which the lander is moving will be stored in variables dx and dy, which will be controlled by the keyboard presses of the user. The new position of the lander after each new frame is drawn will be (x+dx, y+dy).

Movement

The default motion for the craft is downwards due to gravity. During each iteration of the main loop, the craft moves in the increasing y direction, and because it is accelerating the value of the velocity changes too. If y is the current y coordinate and dy is the velocity then:

y = y + dy

dy = dy + 0.5

When the UP key is pressed, the craft should accelerate upwards, and it should continue to accelerate until the key is released. This means adding a value to dy to slow its descent or even make it move upwards. So long as the UP key is depressed, which is to say a KEYUP has not yet been received, the upwards velocity (increase in dy) will continue to increase. When the UP key is depressed a flag is set to true, and when it is released the flag is set to False, and so long as the flag is true we perform dy = dy – 0.3 each iteration. The flag is named power.

Movement to the left and right are done the same way; flags moveleft and moveright indicate the LEFT and RIGHT keys are depressed and motion in that direction should increase; this means changing the value of dx, the velocity in the x direction. Here is the event loop:

for event in pygame.event.get():

if event.type == pygame.KEYDOWN:

if event.key == pygame.K_UP:

power = True

sound.play()

if event.key == pygame.K_LEFT:

moveleft = True

if event.key == pygame.K_RIGHT:

moveright = True

if event.type == pygame.KEYUP:

if event.key == pygame.K_UP:

power = False

sound.stop()

if event.key == pygame.K_LEFT:

moveleft = False

if event.key == pygame.K_RIGHT:

moveright = False

When an engine is turned on fuel is consumed. As long as the UP key is pressed we consume 1 unit of fuel per iteration; sidewise motion uses 0.3 units, but only when the key is pressed. Motion continues regardless because of Newton’s law but needs no fuel. Moving the lander happens after the event loop, and the code looks like this:

if moveleft:

dx = dx - 1

fuel = fuel - 0.3

elif moveright:

dx = dx + 1

fuel = fuel - 0.3

if power:

fuel = fuel - 1

dy = dy - 0.3

else:

dy = dy + 0.5

x = int(x + dx)

y = int(y + dy)

FIGURE 7.20 The background (lunar landscape).

The Background

The lander is drawn over the top of a simple graphic that represents the lunar surface. The lander has landed when it touches the light brown region at the bottom of the image. It is displayed each time through the main loop, and is wider than the drawing window. It is possible to display different portions of it, allowing more levels.

The lander has a special location at the bottom of each of the legs that is used to sense the landing. When the pixel in the drawing area is neither white nor black, then the sensor has touched the ground and the craft has landed.

FIGURE 7.21 The pixels at the ends of the lander legs “sense” the color of the ground.

If the landing has been detected, the game is won if the velocity dy at that time is 3 or less; otherwise, the lander has crashed. A function named landed evaluates whether the craft is down or not.

At this point the rest of the main loop can be provided: we must redraw the background, draw the lander in its proper location, and test to see if it has landed or not.

surf.blit (im, (-100,0))         # Draw background

draw_lander (x,y)         # Draw the lander

if fuel < 0:         # If out of fuel, you lose.

surf.fill ((255,0,0))

lose = True

text("Fuel:"+str(int(fuel)), 10, 10)        # Display fuel

if landed(x, y):         # If landed, how fast?

if dy < 3:         # Slow, good news

win = True

else:         # Too fast. Crash.

lose = True

# Display the altitude and speed.

text ("Speed: "+str( int(dy*100)//100)+

" height "+str(y), 200, 20)

pygame.display.update()

A few other things are done here. The fuel is checked, and if we’re out of fuel the game is over. The flags win and lose indicate that the game is over and the player won or lost, respectively.

The current amount of fuel, altitude, and speed are displayed on the screen using a function named text. It is interesting to examine this function:

def text (s, x, y):

global font

text = font.render(s, 1, (200,200,0))

pygame.draw.rect (surf, (0,0,0), (x, y,200, 20))

surf.blit (text, (x,y))

Here the parameters x and y indicate the location of the text on the screen. The text is rendered into a local Surface and is blitted into the display surface. It is a convenient function to have.

Sound

There is only one sound, that being the engine. When the UP key is pressed, the engine is engaged and makes a noise. The sound is in a file “engine.wav” which is loaded at the beginning of the game. The event loop listed above shows that when the UP key is depressed, there is a call to sound.play(), and when the key is released, there is a call to sound.stop(). It’s basically that simple.

Landing

Detecting a landing has a small set of possibilities. The function landed must return either True or False, and all situations have to be enumerated. Here is the function, annotated:

def landed (x, y):

global im, surf

Parameters x and y define the origin of the lander graphic.

if y>311:

return True

The lander has passed the bottom of the screen.

if y<10:

return False

Lander is above the top of the screen.

if x<6 or x>374:

return False

Lander is off the screen left or right.

c1 = surf.get_at ((x+25, y+26))

if c1[0] != 0 and c1[0] != 255:

return True

If the color of the pixel at the right lander leg is not black or white, then the lander is touching the ground.

c2 = surf.get_at ((x-5, y+26))

if c2[0] != 0 and c2[0] != 255:

return True

If the color of the pixel at the left lander leg is not black or white, then the lander is touching the ground.

return False

Default return value is false.

Improvements

Two important additions could be made to this game. First, when the engine is on, there should be some kind of exhaust visible at the bottom of the lander. This is a perfect situation for a small animation.

Similarly, perhaps if the lander crashes, it should be seen to explode.

Other sounds are possible: the left and right engines possibly, the sound of a successful landing or a crash, and perhaps even some music.

Finally, a start screen showing the rules and controls is always a good idea in a computer game.

Conclusion

Pygame has a lot of features that have not been discussed in this brief chapter, but these can be found in books and in online documentation and tutorial pages. Here is a list of places to begin looking for more information:

http://www.pygame.org/docs/

Online documentation.

http://www.pygame.org/docs/tut/intro/intro.html

Tutorial.

http://www.cogsci.rpi.edu/~destem/gamedev/pygame.pdf

Cheat sheet.

CHAPTER 8

HANDLING DATA

This chapter will be an examination of how certain kinds of data are represented and the consequences insofar as how computer programs can use these data. Python in particular will be used for this examination, although some of the discussion is more general. Of course, the discussion will be driven by practical things and by how things can be accomplished using Python.

Most data consists of measurements of something, and as such are fundamentally numeric. Astronomers measure the brightness of stars, as an example, and note how they vary or not as a function of time. The data consists of a collection of numbers that represent brightness on some arbitrary scale; the units of measurements are always in some sense arbitrary. However, units can be converted from one kind to another quite simply, so this is not a problem. Biologists frequently count things, so again their data is fundamentally numeric. Social scientists ask questions and collect answers into groups, again a numeric result.

Then there are search engines, which can be thought of as an extension of human memory and reasoning. The ability of humans to access information has improved hugely over the past twenty years. If the phrase “python data manipulation” is entered to the Google search engine, over half a million results are returned. True, many may not directly relate to the query as it was intended, but part of the problem will be in the phrasing of the request.

How is all of this done? It does take some clever algorithms and good programming, but it also requires a language that offers the right facilities.

Dictionaries

A Python dictionary is an important structure for dealing with data, and is the only important language feature that has not been discussed until now. One reason is that a dictionary is more properly an advanced structure that is implemented in terms of more basic ones. A list, for example, is a collection of things (integers, reals, strings) that is accessed by using an index, where the index is an integer. If the integer is given, the contents of the list at that location can be retrieved or modified.

A dictionary allows a more complex, expensive, and useful indexing scheme: it is accessed by content. Well, by a description of content at least. A dictionary can be indexed by a string, which in general would be referred to as a key, and the information at that location in the dictionary is said to be associated with that key. An example: a dictionary that returns the value of a color given the name. A color, as described in Chapter 7, is specified by a red, green, and blue component. A tuple such as (100,200,100) can be used to represent a color. So in a dictionary named colors the value of colors[‘red’] might be (255,0,0) and colors[‘blue’] is (0,0,255). Naturally, it is important to know what names are possible or the index used will not be legal and will cause an error. So colors[‘copper’] may result in an index error, which for a dictionary is called a KeyError.

The Python syntax for setting up a dictionary differs from anything that has been seen before. The dictionary colors could be created in this way:

colors = {'red':(255, 0, 0), 'blue':(0,0,255), 'green':(0,255,0)}

The braces { . . . } enclose all of the things being defined as part of the dictionary. Each entry is a pair, with a key followed by a “:” followed by a data element. The pair ‘red’:(255,0,0) means that the key ‘red’ will be associated with the value (255,0,0) in this dictionary.

Now the name colors looks like a list, but it is indexed by a string:

print (colors['blue'])

The index is called a key when referring to a dictionary. That’s because it is not really an index, in that the string can’t directly address a location. Instead the key is searched for, and if it is a legal key (i.e., has been defined), the corresponding data element is selected. The definition of colors creates a list of keys and a list of data:

When the expression colors[‘blue’] is seen, the key ‘blue’ is searched for in the list of all keys. It is found at location 1, so the result of the expression is the data element at 1, which is (0,0,255). Python does all of this work each time a dictionary is accessed, so while it looks simple, it really involves quite a bit of work.

New associations can be made in assignment statements:

colors['khaki'] = (240,230,140)

Indeed, a dictionary can be created with an empty pair of braces and then have values given using assignments:

colors = {}

colors['red'] = (255, 0, 0)

. . .

As with other variables, the value of an element in a dictionary can be changed. This would change the association with the key; there can only be one thing associated with a key. The assignment:

colors['red'] = (200.,0,0)

reassigns the value associated with the key ‘red.’ To delete it altogether use the del() function:

del(colors['blue'])

Other types can be used as keys in a dictionary. In fact, any immutable type can be used. Hence it is possible to create a dictionary that reverses the association of name to its RGB color, allowing the color to be used as the key and the name to be retrieved. For example:

names = {}

names[(255,0,0)] = 'red'

names[(0,255,0)] = 'green'

This dictionary uses tuples as keys. Lists can’t be used because they are not immutable.

Functions for Dictionaries

The power of the store-fetch scheme in the dictionary is impressive. There are some methods that apply mainly to dictionaries and that can be useful in more complex programs. The method keys() returns the collection of all of the keys that can be used with a dictionary. So:

list(dict.keys())

is a list of all of the keys, and this can be searched before doing any complex operations on the dictionary. The list of keys is not in any specific order and if they need to be sorted, then:

sorted(dict.keys())

will do the job. The del() method has been used to remove specific keys, but dict.clear() will remove all of them.

The method setdefault() can establish a default value for a key that has not been defined. When an attempt is made to access a dictionary using a key, an error occurs if the key has not been defined for that dictionary. This method makes the key known so that no error will occur and give a value that can be returned for it; None, perhaps.

dict.setdefault(key, default=None)

Other useful functions include:

dict.copy() – returns a (shallow) copy of dictionary.

dict.fromkeys() –create a new dictionary setting keys and values.

For example, dict.fromkeys( (“one”, “two”), 3) creates {(“one”, 3), (“two”, 3)}

dict.items() –returns a list of dict’s (key, value) tuple pairs.

dict.values() –returns list of dictionary dict’s values.

dict.update(dict2) –adds the key-value pairs from dictionary dict2 to dict.

The expression key in dict is True if the key specified exists in the dictionary dict.

Dictionaries and Loops

Dictionaries are intended for random access, but on occasion it is necessary to scan through parts or all of one. The trick is to create a list from the pairs in the dictionary and then loop through the list. For example:

for (key,value) in dict.items():

print (key, " has the value ", value)

The keys are given in an internal order which is not alphabetical. Yet it is a simple matter to sort them:

for (key,value) in sorted(dict.items()):

print (key, " has the value ", value)

By converting the dictionary pairs in a list, any of the operations on lists can be applied to a dictionary as well. It is even possible to use comprehensions to initialize a dictionary. For example:

d = {angle:sin(radians(angle)) for angle in (0,45.,90., 135., 180.)}

creates a dictionary of the sines of some angles indexed by the angle.

Arrays

For programmers who have used other languages, Python lists have many of the properties of an array, which in C++ or Java is a collection of consecutive memory locations that contain the same type of value. Lists may be designed to make operations such as concatenation efficient, which means that a list may not be the most efficient way to store things. A Python array is a class that mimics the array type of other languages and offers efficiency in storage, exchanging that for flexibility.

Only certain types can be stored in an array, and the type of the array is specified when it is created. For example:

data = array('f', [12.8, 5.4, 8.0, 8.0, 9.21, 3.14])

creates an array of 6 floating point numbers; the type is indicated by the ‘f’ as the first parameter to the constructor. This concept is unlike the Python norm of types being dynamic and malleable. An array is an array of one kind of thing, and an array can only hold a restricted set of types.

The type code, the first parameter to the constructor, can have one of 13 values, but the most commonly used ones will be:

‘b’ A C++ char type

‘B’ A C++ unsigned char type

‘i’: A C++ int type

‘l’: A C++ long type

‘f’: A C++ float type

‘d’: A C++ double type

Arrays are class objects and are provided in the built-in module array, which must be imported:

from array import array

An array is a sequence type, and has the basic properties and operations that Python provides all sequence types. Array elements can be assigned to and can be used in expressions, and arrays can be searched and extended like other sequences. There are some features of arrays that are unique:

frombytes (s)

The string argument s is converted into byte sequences and appended to the array.

fromfile(f, num)

Reads num items from the file object f and appends them. An integer, for example, is one item.

fromlist (x)

Appends the elements from the list x to the array.

tobytes()

Converts the array into a sequence of bytes in machine representation.

tofile(f)

Writes the array as a sequence of bytes to the file f.

In most cases arrays are used to speed up numerical operations, but they can also be used (and will be in the next section) to access the underlying representations of numbers.

Formatted Text, Formatted I/O

There is a generally believed theory among many users of data, including some engineers and financial analysts, that if numbers line up in nice columns then they must be correct. This is obviously not true, but appearances can matter a great deal, and numbers that do not line up properly for easy reading look sloppy and give people the impression that they may not be as carefully prepared as they should have been. The Python print() function as used so far simply prints a collection of variables and constants with no real attention to a format. Each one is printed in the order specified with a space between them. Sometimes that’s good enough.

The Python versions since 2.7 have incorporated a string format() method that allows a programmer to specify how values should be placed within a string. The idea is to create a string that contains the formatted output and then print the string. A simple example is:

s = "x={} y={}"

fs = s.format (121.2, 6)

The string fs now contains “x=121.2 y=6.” The braces within the format string s hold the place for a value. The format() method lists values to be placed into the string, and with no other information given it does so in order of appearance, in this case 121.2 followed by 6. The first pair of braces is replaced by the first value, 121.2, and the second pair of braces is replaced by the second value, which is 6. Now the string fs can be printed.

This is not how it is usually done, though. Because this is usually part of the output process, it is often placed within the print() call:

print ("x={} y={}".format(121.2, 6) )

where the format() method is referenced from the string constant. No actual formatting is done by this particular call, it is merely a conversion to string and a substitution of values. The way formatting is done depends on the type of the value being formatted, the most common types being strings, integers, and floats. An example will be illuminating.

Example: NASA Meteorite Landing Data

NASA publishes a huge amount of data on its web sites, and one of these is a collection of meteorite landings. It covers many years and has over 4800 entries. The task assigned here is to print a nicely formatted report on selected parts of the data. The data on the file has its fields separated by commas, and there are ten of them: name, id, nametype, recclass, mass, Fall, year, reclat, reclong, and GeoLocation. The report requires that the name, recclass, mass, reclat, and reclong be arranged in a nicely formatted set of columns.

Reading the data is a matter of opening the file, which is named “met.txt,” and calling readline(), then creating a list of the fields using split(“,”). If this is done and the fields are simply printed using print(), the result is messy. An abbreviated example is (simulated data):

infile = open ("met.txt", "r")

inline = infile.readline()

while inline !="":

inlist = inline.split(",")

mass = float(inlist[4])

lat = float(inlist[7])

long = float(inlist[8])

print (inlist[0], inlist[3], inlist[4], inlist[7],

inlist[8])

inline = infile.readline()

infile.close()

The result is, as predicted, messy:

Ashdon H5 121.13519985254874 89.85924301385958 -126.27404435776049

Arbol Solo H6 66.94777134343516 25.567048824444797 160.58088365396014

Baldwyn L6 47.6388587105465 -7.708508536783924 -81.22266156597777

Ankober L6 15.265523451122064 -32.01862330869428 102.31244557598723

Ankober LL6 57.584802700693885 -84.85880091616322 106.31130649523368

Ash Creek L6 62.130089525516155 76.02832670618457 -140.03422105516938

Almahata Sitta LL5 30.476879105555653 -12.906745404586 47.411816322674

Nothing lines up in columns, and the numbers show an impossible degree of precision. Also, there should be headings.

The first field to be printed is called name, and it is a string; it is the name of the location where the observation was made. The print statement simply adds a space after printing it, and so the next thing prints immediately following. Things do not line up. Formatting a string for output involves specifying how much space to allow and whether the string should be centered or aligned to the left or right side of the area where it will be printed. Applying a left alignment to the string variable named placename in a field of 16 characters would be done as follows:

‘{:16s}’.format(placename)

The braces, which have previously been empty, contain formatting directives. Empty braces mean no formatting, and simply hold the place for a value. A full format could contain a name, a conversion part, and a specification:

{ [name] [‘!’ conversion] [‘:’ specification] }

where optional parts are in square brackets. Thus, the minimal format specification is ‘{}’. In the example “{:16s}” there is no name and no conversion parts, only a specification. After the ‘:’ is ‘16s,’ meaning that the data to be placed here is a string, and that 16 characters should be allowed for it. It will be left aligned by default, so if placename was “Atlanta,” the result of the formatting would be the string “Atlanta ”, left aligned in a 16-character string. Unfortunately, if the original string is longer than 16 characters, it will not be truncated, and all of the characters will be placed in the resulting string even if it makes it too long.

To right align a string, simply place a “>” character immediately following the “:”. So:

“{:>16s}”.format(“Atlanta”)

would be “ Atlanta.” Placing a “<” character there does a left alignment (the default) and “^” means to center it in the available space. The alignment specifications apply to numbers as well as strings.

The first two values to be printed in the example are the city name, which is in inlist[0], and the meteorite class, which is inlist[3]. Formatting these is done as follows:

s = '{:16s} {:10s}'.format(inlist[0], inlist[3])

Both Strings Will Be Left Aligned

Numeric formats are more complicated. For integers there is the total space to allow, and also how to align it and what to do with the sign and leading zeros. The formatting letter for an integer is “d,” so the following are legal directives and their meaning:

Floating point numbers have the extra issue of the decimal place. The format character is often “f,” but can be “e” for exponential format or “g” for general format, meaning the system decides whether to use “f” or “e.” Otherwise, the formatting of floating point is like that of previous versions of Python and like that of C and C++:

The next three values to be printed are floating point: the mass of the meteorite and the location, as latitude and longitude. Printing each of these as 7 places, 2 to the right of the decimal, would seem to work. Or, as a format: ‘{:7.2f}’.

The solution to the problem is now at hand. The data is read line by line, converted into a list, and then the fields are formatted and printed in two steps:

infile = open ("met.txt", "r")

inline = infile.readline()

print (" Place Class Mass Latitude Longitude")

while inline !="":

inlist = inline.split(",")

mass = float(inlist[4])

lat = float(inlist[7])

long = float(inlist[8])

print('{:16s} {:14s} {:7.2f}'.format(inlist[0],inlist[3],

mass),end="")

print (' {:7.2f} {:7.2f}'.format(lat, long))

inline = infile.readline()

infile.close()

The result is:

There are many more formatting directives, and a huge number of their combinations. Future examples may expose them.

Advanced Data Files

File operations were discussed Chapter 5, but the discussion was limited to files containing text. Text is crucial because it is how humans communicate with the computer; people are unhappy about having to enter binary numbers. On the other hand, text files take up more space than needed to hold the information they do. Each character requires at least one byte. The number 3.1415926535 thus takes up 12 bytes, but if stored as a floating-point number, it needs only 4 or 8 depending on precision.

The file system on most computers also permits a variety of operations that have not been discussed. This includes reading from any point in a file, appending data to files, and modifying data. The need for processing data effectively is a main reason for computers to exist at all, so it is important to know as much as possible about how to program a computer for these purposes.

Binary Files

A binary file is one that does not contain text, but instead holds the raw, internal representation of its data. Of course, all files on a computer disk are binary in the strict sense, because they all contain numbers in binary form, but a binary file in this discussion does not contain information that can be read by a human. Binary files can be more efficient than other kinds, both in file size (smaller) and the time it takes to read and write them (less). Many standard files types, such as MP3, exist as binary files, so it is important to understand how to manipulate them.

Example: Create a File of Integers

The array type holds data in a form that is more natural for most computers than does a list, and also has the tofile() method built in. If a collection of integers is to be written as a binary file, a first step is to place them into an array. If a set of 10000 consecutive integers are to be written to a file named “ints” the first step is to import the array class and open the output file. Notice that the file is open in “wb” mode, which means “write binary”:

from array import array

output_file = open('ints', 'wb')

Now create an array to hold the elements and fill the array with the consecutive integers:

arr = array('i')

for k in range (10000, 20000):

arr.append(k)

Finally, write the data in the array to the file:

arr.tofile(out)

out.close()

This file has a size listed as 40kb on a Windows PC. A file having the same integers written as text is 49kb. This is not exactly a huge space saving, but it does add up.

Reading these values back is just as simple:

inf = open ('ints', 'rb')

arrin = array('i')

for k in range (0, 10001):

try:

arrin.fromfile(inf, 1)

except:

break

print (arrin[k])

inf.close()

The try is used to catch an end-of-file error in cases where the number of items on the file is not known in advance. Or just because always doing so is a good idea.

Sometimes a binary file will contain data that is all of the same type, but that situation is not very common. It is more likely that the file will have strings, integers, and floats intermixed. Imagine a file of data for bank accounts or magazine subscriptions; the information included would be names and addresses, dates, financial values, and optional data depending on the specific situation. Some customers have multiple accounts, for example. How can binary files be created that contain more than one kind of information? By using structs.

The Struct Module

The struct module permits variables and objects of various types to be converted into what amounts to a sequence of bytes. It is a common claim that this is in order to convert between Python forms and C forms, because C has a struct type (short for structure). However, many files exist that consist of mixed type data in raw (i.e., machine compatible) form that have been created by many programs in many languages. It is possible that C is singled out because the name struct was used.

Example: A Video Game High Score File

Video game players need little incentive to try hard to win a game, but for many years a special reward is given to the better players. The game “remembers” the best players and lists them at the beginning and end of the game. This kind of ego boost is a part of the reward system of the game. The game program stores the information on a file in descending order of score. The data that is saved is usually the player’s name or initials, the score, and the date. This mixes string with numeric data.

Consider that the player’s name is held in a variable name, the score is an integer score, and the date is a set of three strings year, month, and day. In this situation the size of each value needs to be fixed, so allow 32 characters for the name, 4 for year, 2 for month, and 2 for day. The file was created with the name first, then the score, then the year, month, and day. The order matters because it will be read in the same order that it was written. On the file the data will look like this:

Each letter in the first string represents a byte in the data for this entry. The “c”s represent characters; the “i”s represent bytes that are part of an integer. There are 44 bytes in all, which is the size of one data record, which is what one set of related data is generally called. A file contains the records for all of the elements in the data set, and in this case a record is the data for one player, or at least one time that the player played the game. There can be multiple entries for a player.

One way to convert mixed data like this into a struct is to use the pack() method. It takes a format parameter first, which indicates what the struct will consist of in terms of bytes. Then the values are passed that will be converted into components of the final struct. For the example here the call to pack() would be:

s = pack ("32si4s2s2s", name, score, year, month, day)

The format string is “32si4s2s2s”; there are 5 parts to this, one for each of the values to be packed:

32s

is a 32-character-long string. It should be of type bytes.

i

is one integer. However, 2i would be two integers, and 12i is 12 integers.

4s

is a 4-character-long string.

2s

is a 2-character-long string.

Other important format items are:

c

is a character

f

is a float

d

is a double-precision float

The value returned from pack() has type bytes, and in this case is 44 bytes long. The high score file consists of many of these records, all of which are the same size. A record can be written to a file using write(). So, a program that writes just one such record would be:

from struct import *

f = open ("hiscores", "wb")

name = bytes("Jim Parker", 'UTF-8')

score = 109800

year = b"2015"

month = b"12"

day = b"26"

s = pack ("32si4s2s2s", name, score, year, month, day)

f.write(s)

Reading this file involves first reading the string of bytes that represented a data record. Then it is unpacked, which is the reverse of what pack() does, and the variables passed to the unpack() function to be filled with data. The unpack() method takes a format string as the first parameter, the same kind of format string as pack() uses. It will return a tuple. An example that reads the record in the above code would be:

from struct import *

f = open("hiscores", "rb")

s = f.read(44)

name,score,year,month,day = unpack("32si4s2s2s", s)

name = name.decode("UTF-8")

year = year.decode("UTF-8")

month = month.decode("UTF-8")

day = day.decode("UTF-8")

The data returned by unpack are bytes, and need to be converted into strings before being used in most cases. Note the input mode on the open() call is “rb,” read binary.

A file in this format has been provided, and is named simply ‘hiscore’. When a player plays the game, they will enter their name; the computer knows their score and the date. A new entry must be made in the ‘hiscore’ file with this new score in it. How is that done?

Start with the new player data for Karl Holter, with a score of 100000. To update the file it is opened and records are read and written to a new temporary file (named ‘tmp’) until one is found that has a smaller score than the 100000 that Karl achieved. Then Karl’s record is written to the temporary file, and the remainder of ‘hiscores’ is copied there. This creates a new file named ‘tmp’ that has Karl’s data added to it, and in the correct place. Now that file can be copied to ‘hiscores’ replacing the old file, or the file named ‘tmp’ can be renamed as ‘hiscores’. This is called a sequential file update.

Renaming the file requires access to some of the operating system functions in the module os; in particular:

os.rename ("tmp", "hiscores")

Random Access

It seems natural to begin reading a file from the beginning, but that is not always necessary. If the data that is desired is located at a known place in the file, then the location being read from can be set to that point. This is a natural consequence of the fact that disk devices can be positioned at any location at any time. Why not files too?

The function that positions the file at a specific byte location is seek():

f.seek(44)        # Position the file at byte 44,

# which is the second record in the hiscores file.

It’s also possible to position the file relative to the current location:

f.seek(44, 1) # Position the file 44 bytes from this location,

# which skips over the next record in hiscores.

A file can be rewound so that it can be read over again by calling f.seek(0), positioning the file at the beginning. It is otherwise difficult to make use of this feature unless the records on the file are of a fixed size, as they are in the file ‘hiscores’, or the information on record sizes is saved in the file. Some files are intended from the outset to be used as random access files. Those files have an index that allows specific records to be read on demand. This is very much like a dictionary, but on a file. Assuming that the score for player Arlen Franks is needed, the name is searched for in the index. The result is the byte offset for Arlen’s high score entry in the file.

Arlen’s record starts at byte 352 (8th record * 44 bytes). He just played the game again and improved his score. Why not update his record on the file? The file needs to be open for input and output, so mode “rb+”, meaning open a binary file for input and output, would work in this case. Then position the file to Arlen’s record, create a new record, and write that one record. This is new—being able to both read and write the same file seems odd, but if the data being written is exactly the same size as the record on the file, then no harm should come from it. The program is:

# read and print hiscore file

from struct import *

f = open ("hiscores", "r+b") # Open binary file,input and output

pos = 44*8         # Desired record is 8, 44 bytes per

f.seek(pos)         # Seek to that position one the file

s = f.read(44)        # Read the target record

name = b'Arlen Franks'         # Make a new one with a new score

score = 100300

year = b'201 5'

month = b'12'

day = b'26'         # Pack the new data

ss = pack("32si4s2s2s", name,score, year,month,day)

f.seek (44*8)        # Seek the original position again!

f.write(ss)         # Write the new data over the old

f.close ()         # Close the file

This works fine, provided that the position of Arlen’s data in the file is known. It does not maintain the file in descending order, though.

Example: Maintaining the High Score File in Order

The circumstances of the new problem are that a player only appears in the high score file once and the file is maintained in descending order of score. If a player improves their score, then their entry should move closer to the beginning of the file. This is a more difficult problem than before, but one that is still practical. So, presume that a player has achieved a new score. The entire process should be:

Get the player’s old score.

Read the file, get the player’s record, unpack it.

Is the new score larger?

If not, close the file. Done.

Yes, so find out where the score belongs, in the file.

Look at successively preceding records until one is found that has a larger score.

Place the new record where it belongs.

Copy the records from the new position for the record ahead one position until the old position is reached.

The process is like moving a playing card closer to the top of the deck while leaving the other cards in the same order. It’s probably more efficient to move the record while searching for the correct position, though. Each time the previous record is examined, if it does not have a larger score, then the record being placed is copied ahead one position. This results in a pretty compact program, given the nature of the problem, but it is a bit tricky to get right. For example, what if the new score is the highest? What if the current high score gets a higher score?

CHAPTER 9

COMMUNICATION USING PYTHON

In the age of high speed Internet, social media, podcasts, blogs, and wikis, input from the keyboard and output to the screen is not enough. The wide world outside of the desktop beckons, and a programmer with a knowledge of Python and the relevant modules can respond.

Can a computer communicate with another one? Of course. Can a program send Email? Yes, that’s what a mail program like Thunderbird or Outlook does. Can a program be written that reads tweets as they are sent? Sure, but there is a price. That is: these things are done according to someone else’s rules. The first Email was sent in 1971 on a private network named Arpanet. It sent mail between distinct computers, rather than sending messages between users on a specific machine. In 1972 Unix Email was made available, and was networked in 1978; that was the start of something big.

The sender and receiver had to agree on how to encode and decode a message, and how to access it from the network. To send mail between different computers always requires a standard, a scheme that is agreed upon by implementers of the system. Otherwise, mail can only be sent between UNIX systems, or Windows, or iOS. Email, to be practical, needs to be more flexible. It needs to be ubiquitous, and so all need to agree on a standard for how Email can be sent and received. A standard was eventually agreed on, and it was called the Simple Mail Transfer Protocol (SMTP) and was established in 1982.

This was seven years before the World Wide Web, so Email really represents the first practical way to communicate between computers over a long distance. FTP happened at about the same time. The enabling technology for the Web, TCP/IP, came next. All of these developments in networking and software combined to create the modern interconnected society, but all are based on a collection of rules that software must agree to (protocols) if they are to make use of the network infrastructure. This is an example of design by contract, in which designers create formal specifications for components, and using those involves a kind-of contract or agreement between programmers developing client software and those who built the modules and designed the protocols.

There are high-level programs that provide a good user interface to the Internet and that implement these protocols beneath their visual presentation. When using Python a collection of modules are used that handle the very low-level details, but the interface to the programmer exposes the protocol. Some of these modules are provided in a standard Python installation (smtplib, email), and some are not (MPI, Tweepy) and will have to be installed before the code in this chapter will run.

When communicating with another machine, a key issue is that of authentication. Almost all protocols require that a connection be formed between the two computers, using some kind of identification of those machines such as their IP address. Then the one initializing the connection must prove that it has permission to do what it is about to do. This resembles logging in, and involves a user identification and a password of some type. Once the user has been identified, there is an exchange of messages that tell the remote computer what is desired of it, and that allow information to be returned to the caller. This process is nearly universal, but it takes somewhat different forms on different systems.

Email

Email is a good example of a client-server system, and one that gets used millions of times each minute. The Email program on a PC is the client, and allows a user to enter text messages, specify destinations, attach images, and all of the features expected by such a program. This client packages the Email message (data) according to the Simple Message Transfer Protocol (SMTP) and sends that to another computer on the Internet, the Email server. An Email user must have an account on the server for this to work so they can be identified and the user can receive replies; so, the process is: log into the Email server, then send the SMTP message to the Email server program on that server. Thus the client side of the contract is to create a properly formatted message, to log into the server properly, and pass the message to it.

Now the server does the work. Given the destination of the message, it searches for the server that is connected to that destination. For example, given the address xyz@gmail.com, the server for gmail.com is located. Then the Email message is sent across the network to that server. The server software at that end reads the message and places it into the mailbox, which is really just a directory on a disk drive connected to the server, for the specified use xyz. The mail message is essentially a text file at this point.

This description is simplified but essentially accurate, and describes what has to be done by a program that is supposed to send an Email message. The Python module that permits the sending of Email implements the protocol and offers the programmer ways to specify the parameters, like the destination and the message. The interface is implemented as a set of functions. The library needed for this is smtplib, a part of the standard Python system.

Example: Send an Email

Sending an Email message starts with establishing a connection between the client computer and the user’s mail server, the one on which they have an account (user name and password). For the purposes here a Gmail (Google) server will be used, which complicates the issue a tiny bit. The Email accounts in the example are also Gmail ones, and these can be had for free from Google.

The program must declare smtplib as an imported module. The sending address and the receiving address will be the same in this example, but this is just a test. Normally this will not be the situation. The Email address is the user ID for Gmail authentication and the password is defined by the user. These are all just strings.

import smtplib

LOGIN = yourloginID # Login User ID for Gmail, string

PASSWD = yourpassword # Login password for Gmail, string

sndr = pythontextbook@gmail.com # Sender's email address

rcvr = pythontextbook@gmail.com # Receiver's email address

Part of the SMTP scheme is a syntax for Email messages. There is a header at the beginning that specifies the sender, receiver, and subject of the message. These are used to format the message, not to route it—the receiver address is specified later. A simple such message looks like this:

From: user_me@gmail.com

To: user_you@gmail.com

Subject: Just a message

A string must be constructed that contains this information:

msgt = "From: user_me@gmail.com\n"

msgt = msgt + "To: user_you@gmail.com\n"

msgt = msgt + "Subject: Just a message\n"

msgt = msgt + "\n"

Now the body of the message is attached to this string. This is the part of the Email that is important to the sender:

msgt = msgt + "Attention: This message was sent by Python!\n"

The string variable msgt now holds the whole message. This message is in the format defined by the Multipurpose Internet Mail Extensions (MIME) standard. The next step for the program is to try to establish a connection with the sender’s Email server. For this the smtp module is needed, specifically the SMTP() function. It is called passing the name of the user’s Email server as a parameter, and it returns a variable that references that server. In this example that variable is named server:

server = smtplib.SMTP('smtp.gmail.com')

If it is not possible to connect to the server for some reason, then an error will occur. It is therefore a good idea to place this in a try-except block:

try:

server = smtplib.SMTP('smtp.gmail.com')

except:

print ("Error occurred. Can't connect")

else:

Now comes the complexity that Gmail and some other servers introduce. What has happened after the call to smtplib.SMTP() is that a communications session has been opened up. There is now an active connection between the client computer and the server at smtp.gmail.com. Some servers demand a level of security that, among other things, ensures that other parties can’t modify or even read the message. This is accomplished using a protocol named Transport Layer Security (TLS), the details of which are not completely relevant because the modules take care of it. However, to send data to smtp.gmail.com, the server must be told to begin using TLS:

server.starttls()

Now the user must be authenticated using their ID and password:

server.login(LOGIN,PASSWD)

Only now can a message be sent, and only if the login ID and password are correct. The sender is the string sndr, the recipient is rcvr, and the message is msgt:

server.sendmail(sndr, rcvr, msgt)

Now that the message has been sent, it is polite to close the session. Logging off of the server is done as follows:

server.quit()

This program will send one Email, but it can be easily modified to send many Emails one after the other. It can be modified to read the message from the keyboard, or perform any of the functions of a typical Email-sending program.

The module Email can be invoked to format the message in MIME form. The function MIMEText(s) converts the message string s into an internal form, which is a MIME message. Fields like the subject and sender can be added to the message, and then it is sent as was done before. For example:

import smtplib

from email.mime.text import MIMEText

LOGIN = yourloginID

PASSWD = yourpassword

fp = open ("message.txt", "r")        # Read the message from a file

mtest = fp.read()

# Or: simply use a string

#mtest = "A message from Python: Merry Christmas."

fp.close()

msg = MIMEText (mtest)         # Create a MIME string

sndr = pythontextbook@gmail.com         # Sender's Email

rcvr = pythontextbook@gmail.com         # Recipient's Email

msg['Subject'] = 'Mail from Python'        # Add Subject to

the message

msg['From'] = sndr         # Add sender to the message

msg['To'] = rcvr         # Add recipent to the message

# Send the message using Google's SMTP server, as before

s = smtplib.SMTP('smtp.gmail.com') # localhost could work

s.starttls()

s.login (LOGIN, PASSWD)

s.send_message(msg)

s.quit()

Using MIMEText() to create the message avoids having to format it correctly using basic string operations.

Figure 9.1 outlines the procedure for sending an Email using Python.

FIGURE 9.1 Reading and sending Email using Python.

Reading Email

Reading Email is more complicated than writing it. The content of an Email is often a surprise, and so a reader must be prepared to parse anything that might be sent. Which mailbox will be looked at, and when a user reads Email their mailbox often has more than one message in it; how can they be distinguished? In addition, the protocol for retrieving mail from a server is different from that used to send it; in fact there are two competing protocols: POP and IMAP.

The Post Office Protocol (POP) is the older of the two schemes, although it has been updated a few times. It certainly allows the basic requirements of a mail reader, which is to download and delete a message in a remote mailbox (i.e., on the server). The Internet Message Access Protocol (IMAP) is intended for use by many Email clients, and so messages tend not to be deleted until that is requested. When setting up an Email client, one of these protocols usually has to be specified, and then it will be used from then on. The example here will use IMAP.

Example: Display the Subject Headers for Emails in Inbox

An outline for the process of reading Email is sketched on the right-hand side of Figure 13.1. Reading Email uses a different module that was used to send Email: imaplib, for reading from an IMAP server. The function names are different from those in smtplib, but the purpose of some of them is the same. The first three steps in reading Email are:

import imaplib

server = 'imap.gmail.com'         # Gmail's IMAP server

USER = 'your email'               # User ID

PASSWORD = "password"             # Mask this password

EMAIL_FOLDER = "Inbox"

mbox = imaplib.IMAP4_SSL(server)        # Connect to the server

mbox.login(USER, PASSWORD)         # Authenticate (log in)

The next step is to select a mailbox to read. Each has a name, and is really just a directory someplace. The variable mbox is a class instance of a class named imaplib.IMAP4_SSL, the details of which can be found in many places including the Internet. It has a method named select() that allows the examination of a mailbox, given its name (a string). The string is a variable named EMAIL_FOLDER which contains “Inbox,” and the call to select() that essentially opens the inbox is:

z = mbox.select(EMAIL_FOLDER)

The return value is a tuple. The first element indicates success or failure, and if z[0] contains the string “OK,” and then the mailbox is open. The usual alternative is “NO.” The second element of the tuple indicates how many messages there are, but it is in an odd format. If there are 2 messages, as in the example, this string is b’2’; if there were 3 messages it would be b’3’; and so on. These are called message sequence numbers.

Having opened the mailbox, the next step is to read it and extract the messages. The protocol requires that the mailbox be searched for the messages that are wanted. The imaplib.IMAP4_SSL class offers the search() method for this, the simplest form being:

mbox.search(None, "ALL")

which returns all of the messages in the mailbox. IMAP provides search functionality, and all this method does is connect to it, which is why it seems awkward to use. The first parameter specifies a character set, and None allows it to default to a general value. The second parameter specifies a search criterion as a string. There are dozens of parameters that can be used here and the documentation for IMAP should be examined in detail for solutions to specific problems. However, some of the more useful tags include:

ANSWERED: Messages that have been answered.

BCC <string>: Messages with a specific string in the BCC field.

BEFORE <date>: Messages whose date (not time) is earlier than the specified one.

HEADER <field-name> <string>: A specified field in the header contains the string.

SUBJECT <string>: Messages that contain the specified string in the SUBJECT field.

TO <string>: Messages that contain the specified string in the TO field.

UNSEEN: Messages that do not have the \Seen flag set.

So, a call to search() that looks for the text ‘Python’ in the subject line would be:

mbox.search(None, "SUBJECT Python")

The search() function returns a tuple again, where the first component is a status string (i.e., “OK,” “NO,” “BAD”) and the second is a list of messages satisfying the search criteria in the same format as before. If the second message is the only match, this string will be b’2’. If the first three match it will be b’1 2 3’.

Finally, the messages are read, or fetched. The imaplib.IMAP4_SSL class has a fetch() method to do this, and it again takes some odd parameters. What a programmer thinks of the interface or the API or, in other words, the contract, is not important. What must be done is to satisfy the requirements and accept the data as it is offered. The fetch() method accepts two parameters: the first is the indication of which message is desired. The first message is b’1’, the second is b’2’, and so on. The second parameter is an indicator of what it is that should be returned. The header? If so, pass ‘(RFC822.HEADER)’ as the parameter. Why? Because they ask for it. RFC822 is the name of a protocol. If the Email body is wanted then pass ‘(RFC822.TEXT)’. A short list of possibilities is:

RFC822

- Everything

RFC822.HEADER

- No body, header only

RFC822.TEXT

- Body only

RFC822.SIZE

- Message size

UID

- Message identifier

Multiple of these specifiers can be passed; for example:

mbox.fetch(num, '(UID RFC822.TEXT RFC822.HEADER)')

returns a tuple having three parts: the ID, the body, and the header. By the way, the header tends to be exceptionally long, 40 lines or so, and is mostly uninteresting to a specific application. For this example, the only part of the header that is interesting is the “Subject” part. Fields in the header are separated by the characters ‘\r\n’ so they are easy to extract in a call to split(). Eliminating the header data for a moment, the call:

(env, data) = mbox.fetch(num, '(UID RFC822.TEXT)')

results in a tuple that has an “envelope” that should indicate “OK” (the env variable). The data part is a string that contains the UID and the text body of the message. For example:

[(b'2 (UID 22 RFC822.TEXT {718}', b"Got a collection of old 45's for sale. Contact me.\r\n\r\n-- \r\n"), b')']

This says that this is message 2 and shows the text of that message.

This example is supposed to print all of the subject headers in this mailbox. The call to fetch() should extract the header only:

(env, data) = mbox.fetch(num, '(RFC822.HEADER)')

The details of IMAP are complex enough that it is easy to forget what the original task was, which was to print the subject lines from the messages in the mailbox. All of the relevant methods have been described and completing the program is possible. The entire program is:

import imaplib

server = 'imap.gmail.com'           # IMAP Server

USER = "your email"           # USER ID

PASSWORD = ""           # Mask this password

EMAIL_FOLDER = "Inbox"           # Which mailbox?

mbox = imaplib.IMAP4_SSL(server)           # Connect

mbox.login(USER, PASSWORD)           # Authenticate

env, data = mbox.select(EMAIL_FOLDER)          # Select the mailbox

if env == 'OK':           # Did it work?

print ("Printing subject headers: ", EMAIL_FOLDER)

env, data = mbox.search(None, "ALL")

# Select the messages wanted.

if env != 'OK':           # Are there any?

print ("No messages.", env)          # Nope.

exit()

for num in data[0].split():

# For each selected message b'1 2 3 ...'

(env, data) = mbox.fetch(num, '(RFC822.HEADER)')

# Read it

if env != 'OK':

print ("ERROR getting message", num, ", ", env)

break

s = str(data[0][1])

# Look for the string "Subject" in the header

k = s.find("Subject")

if (k>=0):           # Found it?

s = s[k:]          # Extract the string to the next '\r'

k = s.find('\\r')

s = s[:k]

print (s)           # And print it.

mbox.close()

else:

print ("No such mailbox as ", EMAIL_FOLDER)

mbox.logout()

Typical output would be:

Printing subject headers: Inbox

Subject: Contents of Chapter 13

Subject: 45 RPM

Subject: another Email

The point of this section was to demonstrate how a Python program, or any program for that matter, must comply with external specifications when interfacing with sophisticated software systems, and to introduce the concept of a protocol, a contract between developers. Of course a program that can send Email is useful by itself.

Communication Between Processes

Underneath the FTP and Email protocols, which allow interfaces to applications, lies a communications layer, the programs that actually send bytes between computers or between programs on the same computer. It is conducted very much like a conversation. One person, the client, initiates the conversation (“Hi there!”). The other (the server) responds (“Hello. Nice to see you.”). Now it is the client’s turn again. They take turns sending and accepting messages until one says “goodbye.” These messages might contain Email, or FTP data, or TV programs. This layer does not care what the data is; none of its business, really. Its job is to deliver it.

Data are delivered in packets, each containing a certain amount. In order for the client to deliver the data, there must be a server willing to connect to it. The client needs to know the address of a server, just as an FTP address or Email destination was required before, but now all that is needed is the host name and a port number. A port is really a logical construction, something akin to an element of a list. If two programs agree to share data by having one of them place it in location 50001 of a list and the other one read it from there, it gives an approximate idea of what a port is. Some port numbers are assigned and should not be used for anything else; FTP and Email have assigned ports. Others are available for use, and any two processes can agree to use one.

A module named socket, based on the interprocess communication scheme on UNIX of the same name, is used with Python to send messages back and forth. To create an example two computers should be used, one being the client and one the server, and the IP address of the server is required too.

Example: A Server That Calculates Squares

The client will open a communications link (socket) to the server, which has a known IP address. The server will engage in a short handshake (exchange of strings) and then expect to receive a number for the client. The client will send an integer, the server will receive it, square it, and send back the answer. This simple exchange is really the basis for all communications between computers: one machine sends information, the other receives it, processes it, and returns a reply based on the data it received.

The client: will begin the conversation. It creates a connection, a socket, to the server using the socket() function of the socket module. Protocols must be specified, and the most common ones will be used:

import socket

HOST = '19*.***.*.***'           # The remote host

PORT = 50007           # The same port as used by the server

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

s.connect((HOST, PORT))

Port 50007 is used because nothing else is using it. Now the client starts the conversation, just as it appears at the beginning of this section:

s.send(b'Hi there!')

The send() function sends the message passed as a parameter. The string (as bytes) is transmitted to the server through the variable s, which represents the server. The client now waits for the confirmation string from the server, which should be “Hello. Nice to see you.” The client calls:

data = s.recv(1024)

which waits for a response from the server. This response will be 1024-bytes long at most, and it will wait only for a short time, at which point it will give up and an error will be reported. When this client gets the response, it proceeds to send numbers to the server. They are converted into the bytes type before transmission. In this example it simply loops through 100 consecutive integers:

for i in range (0, 100):

data = str(i).encode()

s.send (data)

After sending to the server it waits for the answer. Actually that’s a part of the receive function:

data = s.recv(1024)

after 100 integers the loop ends and the connection is closed:

s.close()

The Server: is always listening. It creates a socket on a particular port so that the operating system knows something is possible there, but because the server cannot predict when a client will connect or what client it will be, it simply listens for a connection, by calling a function named listen():

import socket

from random import *

HOST = '' # A null string is correct here.

PORT = 50007

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

s.bind ((HOST, PORT))

s.listen()

AF_INET and SOCK_STREAM are constants that tells the system which protocols are being used. These are the most common, but see the documentation for others. The bind() and the listen() functions are new. Associating this connection with a specific port is done using bind(). The tuple (HOST, PORT) says to connect this host to this port. The empty string for HOST implies this computer. The listen() call starts the server process, this program, accepting connections when asked. A process connecting on the port that was specified in bind() will now result in this process, the server, being notified. When a connection request occurs, the server must accept it before doing any input or output:

conn, addr = s.accept()

In the tuple (conn, addr) that is returned, conn represents the connection, like a file descriptor returned from open(), and is used to send and receive data; addr is the address of the sender, the client, and is a string. If the addr were printed:

print ("Connected to ", addr)

It would look like an IP address:

Connected to 423.141.12.911

Now the server can receive data across the connection, and does so by calling recv():

data = conn.recv(1024)

print ("Server heard '", data, "'")

The parameter 1024 specifies the size of the buffer, or the maximum number of bytes that can be received in one call. The variable data is of type bytes, just as the parameter to send() was in the client. The client was the first to send, and it sent the message “Hi there!” That should be the value of data now, if it has been received properly. The response from the server should be “Hello, nice to see you.”:

conn.send (b'Hello. Nice to see you.')

The same connection is used for sending and receiving.

Now the real data gets exchanged. The server will accept integers, sent as bytes. It will square them and transmit the answer back.

while True:

data = conn.recv(1024)          # Read the incoming data

if data:

i = int(data)          # Convert it to integer

print ("Received ", i)

data = str(i*i).encode()         # Square it and

convert to bytes

conn.send (data)          # Send to the client

The server can tell when the connection is closed by the client, but it is also polite to say “Goodbye” somehow, perhaps by sending a particular code. If the loop ever terminates, the server should close the connection:

conn.close()

This is a pretty good example of a data exchange and a contract, because there are specified requirements for each side of this conversation which will result in success if done correctly and failure if messed up. Failure is sometimes indicated by an error message, often a timeout where the client or server was expecting something that never arrived. In other cases failure is not formally indicated at all; the program simply “hangs” there and does nothing. If at any time both processes are trying to receive data, then the program will fail.

Figure 9.2 shows the communication between the client and the server as a diagram. If the client and the server are at any time both trying to accept data from the connection, then the program will fail. In the diagram all data transfers can be seen as transmit-accept pairs between the two processes, and as read-write pairs within the server and write-read pairs within the client.

The FTP protocol can now be seen as a socket connection, wherein the client sends strings (commands) to the server, which parses them, carries out the request, and then sends an acknowledgment back.

# The client

import socket

# The remote host

HOST = '19*.***.*.***'

# The same port used by the server

PORT = 50007

s = socket.socket(socket.AF_INET,\

socket.SOCK_STREAM)

s.connect((HOST, PORT))

s.send(b'Hi there!')

data = s.recv(1024)

for i in range (0, 100):

data = str(i).encode()

s.send (data)

data = s.recv(1024)

s.close()

# The server

import socket

HOST = '' # A null string is ok here.

PORT = 50007

s = socket.socket(socket.

AF_INET, \

socket.SOCK_STREAM)

s.bind ((HOST, PORT))

s.listen()

conn, addr = s.accept()

data = conn.recv(1024)

print ("Server heard '", data, "'")

conn.send (b'Hello. Nice

to see you.')

while True:

# Read the incoming data

data = conn.recv(1024)

if data:

# Convert it to integer

i = int(data)

print ("Received ", i)

# Square it and convert to bytes

data = str(i*i).encode()

# Send to the client

conn.send (data)

conn.close()

FIGURE 9.2 Typical communication between the client and the server processes

INDEX

A

accessor method, 115

addend() function, 75

advanced data files

binary files, 167–169

random access, 171–173

struct module, 169–171

AF_INET, 187

aliasing, 81

alpha, 141

any function, 85

appending data to a file, 109–110

append method, 57

append mode, 99–100

arrays, 161–162

features of, 162

authentication, 176, 177

B

background, 154–155

binary files, 167–169

bind() function, 187

blitting, 139–140

break statement, 30, 31

buffering, 101

buffer interface, 46

built-in functions, for tuples, 53–54

button, 148

bytes, 45–46

C

cannonball class, 121–125

cannon class, 121–125

capitalize(), 43

carling, 90

cat-a-pult, 120–121

cannonball and cannon classes, 121–125

circumference of any circle (input), computing, 9–10

class

cannonball and cannon, 121–125

constructor, 112, 113

deck of cards game, 119–120

duck typing, 129–130

encapsulation, 117–118

inheritance, 125–129

methods, 113–114

overview, 111–112

point, 114–118

Python, 112–114

really simple, 114–117

subclasses, 125–129

triangle, 116–117

client, 186

communication between server and, 89, 188

clock.tick(30), 146

close() operation, 106

close the file, 98

color gradient, creating, 135–136

colors, 133–134, 158–159

graphics windows and, see graphics windows and colors

parameters, 138

Comma Separated Variable (CSV), 103–106

communication between processes, server to calculates squares, 185–189

concatenate, 40

constructor, 112, 113

continue statement, 30

copy a file to another, 108

count, 43, 61

craps game, 66–67

CSV, see Comma Separated Variable

curves, 136–137

D

data

arrays, 161–162

binary files, 167–169

dictionaries, see dictionaries

formatted I/O, 162–165

formatted text, 162–165

left aligned strings, 165–167

random access, 171–173

struct module, 169–171

data files, 95

data record, 169

decode() method, 45

default value, 77

default parameters, 76–78

deleting an element, 50

del() method, 160

destructor, 116

dict.copy(), 160

dict.fromkeys(), 160

dictionaries, 158–159

functions for, 160

and loops, 160–161

dict.items(), 160

dict.update(dict2), 160

dict.values(), 160

difference in sets, 65

displayState() function, 79

display surface, 133

distance method, 115

docstring, 15

documentation, 14–15

dodge() method, 128

drawing a circle using characters, 7

drawing text, 140–141

draw() method, 129

duck typing, 129–130

E

ellipse method, 138

else clause

in a for loop, 31

in a while loop, 31

else statements, 13–14, 17–18

Email, 176–177

display the subject headers in inbox, 181–185

history of, 175

reading, 181–185

sending, 177–180

end of file condition, 98, 101–102

endswith, 43

EOFError, 102

equality in sets, 65

error handler, 33

errors, 32–34

events, 146–147

exceptions, 32–34, 63

delete a specified element from a list, 63–64

File Not Found, 100

except statement, 33–34

executable files, 95

executing Python, 1–3

extend method, 58

external documentation, 14

F

failure, 188

fetch() method, 183

file descriptor, 99

FileNotFound error, 100, 107

file of integers, creating, 168–169

files using in Python

appending data to a, 109–110

close the file, 98

Comma Separated Variable (CSV), 103–106

common file input operations, 102–103

open a file, 98–100

reading from, 98, 100–102

with statement, 106–107

types, 95–96

writing, 98, 107–109

find, 43

fire() method, 123

float, 8

floating-point, 9

floating point numbers, 161, 165–166

fname, 40–41

for loops, 27, 28, 30, 31, 62, 148, 150, see also while loops

counting, 25–27

tuples in, 47–49

using strings, 44–45

format() method, 162–163

formatted I/O, 162–165

formatted text, 162–165

formatting floating point, 166–167

frombytes (s), 162

fromfile(f, num), 162

fromlist (x), 162

fstring, 83

FTP protocol, 185, 187

functions

addend(), 75

to calculate square root of its parameter, 73–74

creating a Python module, 91–93

definition of, 69–70

defult parameters, 76–78

displayState(), 79

execution, 71–74

finding the maximum value of, 86–88

gameOver(), 80

getComputerMove(), 81, 82

getData(), 105

getMove(), 80, 82

gothere, 76–77

max(), 92

maxr(), 92

nextRecord(), 105

None value, 78

parameters, 74–76

print(), 83

printFun, 85–86, 89

readline(), 101, 103

recursion, 89–91

as return values, 88–89

roote, 73

searchr(), 90

square, 74

str(), 60

variables as, 85–88

function type(), 19

G

game, 151

gameOver() function, 80

getComputerMove() function, 81, 82

getData() function, 105

getMove() function, 80, 82

getx() method, 130

global variable, 81–83

Gmail (Google) server, 177

gobject superclass, 126–128

gothere function, 76–77

graphics windows and colors, 132–133

blitting, 139–140

drawing text, 140–141

lines and curves, 136–138

pixel-level, 134–136

polygons, 138–139

Pygame, see Pygame

simple static drawing, 133–134

H

handle, 99

“hangs”, 188

high score file

maintaining, 173

video game, 169–171

HOST, 187

I

IDE, see integrated development environment

if statements, 10–13, 17–18, 21, 27

image files, 95

images, 142–143

imaplib.IMAP4_SSL class, 181–183

IMAP server, 181, 182, 184

immutable tuples, 50, 51

import max, 92

import statement, 25

inbox, display the subject headers in, 181–185

index method, 59

infile variable, 99, 100–101

inheritance, 125–129

objects in video game, 126–129

insert method, 57

int(input()), 10

integers, 7–8, 19, 165

integrated development environment (IDE), 2–3

intersection in sets, 64

__init__ method, 113, 118, 126–127

introduce() method, 113–114

IOError, 100

IP address, 176, 185, 187

isdigit(), 43

isinstance() function, 128

islower(), 43

isprime variable, 27–29

isspace(), 43

isupper(), 44

K

key, 158–159

keyboard, 149–150

KeyError, 158

keys() method, 160

L

lander, 151–152

landing, 155–156

left aligned strings, 165–167

linear programming, 86

line() function, 136

lines and curves, 136–138

list comprehension, 61–62

listen() function, 187

lists, 54–56, 158, 161

computing average (mean) of a list of numbers, 55–56

editing, 56–57

append, 57

count, 61

extend, 58

index, 59

insert, 57

methods, 56–57

remove, 58

reverse, 60–61

sort, 59–60

and tuples, 62–63

variable parameter, 83–84

local variable, 81

log file, 109

loops

for, see for loops

dictionaries and, 160–161

while, see while loops

lower(), 42–44

M

max() function, 92

maxr() function, 92

mbox variable, 181

MIME standard, see Multipurpose Internet Mail Extensions standard

MIMEText(), 179, 180

mobject subglass, 127, 128

modules, 23–25

creating, 91–93

mouse, 147–148

mouse.get_pos(), 147

movement, 152–154

move() methods, 117, 128, 129

Multipurpose Internet Mail Extensions (MIME) standard, 178–179

mutators, 115

N

named parameters, 84

nextRecord() function, 105

**nlist, 84

None value, 78

Game of Sticks, 79–81

non-prime numbers, 27–29, 30

number base, 8–10

O

object reference, 74, 76

open(), 107

open a file, 98–100

OSError, 100

P

pack() method, 170

parameters, 74–76

defult, 76–78

password, 177

patterns, 102–103

2*pi*r expression, 74

pixel, 134–136, 143–145

point class, 114–118, 129

polygons, 138–139

port, 185, 187

poundn, 71

prime numbers, 27–29

print directive, 7

print() function, 19, 83, 162

printFun function, 85–86, 89

private class, 118

prompt, 14

protected variable, 118

PyCharm, 3

Pygame

color class, 134

create a page of notepaper, 134–135

create games, 151–156

creating computer graphics, 132–133

drawing a notepaper, 136–138

installing, 132

mixer, 150

pixel, 143–145

sound, 150–151, 155

user interactions between frames, 45–150

pygame.event.get(), 147

pygame.font.Font, 141

pygame.image.load, 143

Python Graphical User Interface (GUI), 2

R

randint function, 25

random access, 171–173

random module, 25

random numbers, 23–25

randrange(), 67

ranges, 26–27

reading Email, 181–185

reading from files, 98, 100–102

readline() function, 101, 103

really simple class, 114–117

real numbers, 8

rect method, 138

recursion, 89–91

remove method, 58

replace, 44

return statement, 71

return values, functions as, 88–89

reverse method, 60–61

RGB, 133

RGBA, 141

Rock-Paper-Scissors, 3

problem solving, 15–18

exchanging information with computer, 6–7

number bases, 8–10

strings, integers, and real numbers, 7–8

variables and values, experimenting with GUI, 4–6

roote function, 73

S

SCRALPHA property, 141

search() method, 182

searchr() function, 90

self.name, 113

semantics, 112–114

send() function, 186

sending Email, 177–180

sequential file update, 171

server, 186–187

communication between client and, 188, 189

setdefault() method, 160

setters, 115

set types, 64–65

Simple Message Transfer Protocol (SMTP), 175, 176

simple static drawing, 133–134

singletons, 83

slicing, strings, 39–40

s.lower(), 43

s1.lower(), 43

SMTP, see Simple Message Transfer Protocol

SMTP() function, 178

smtp.gmail.com, 178–179

smtplib module, 177, 181

smtplib.SMTP(), 178

socket() function, 185–186

SOCK_STREAM, 187

sort method, 59–60

sound, 150–151, 155

files, 95

spanning multiple lines, 44

split, 44

split(), 104

splitlines, 44

sqrt(64), 25

square function, 74

start_angle, 137

statements, 5

step() method, 121, 123, 124

stop_angle, 137

stretchy lines, 149

str() function, 60

strings, 7, 35–36, 45, 55

change the suffix of a file name, 41

characters, 36–37

comparing, 37–38

constants, 36, 45

create a JPEG file name from a basic string, 40–41

editing, 40–42

extracting parts of, 39–40

identify a “print” statement in, 39–40

leading spaces, 37

left aligned, 165–167

length, 36

for loops using, 44–45

methods, 42–44

Python, 116

reverse the order of characters in a string, 41–42

slicing, 39–40

struct module, 169–171

subclasses, objects in video game, 126–129

subset, 64

surface, 132–133

surf parameters, 137, 138

surf variable, 133, 134

syntax, 112–114

T

table of squares file, 110

write a, 107–109

testCollision() method, 121, 124

text files, 95, 96

thresholding, 144–145

time, 146

timeout, 188

tkinter, 131

TLS, see Transport Layer Security

tobytes(), 162

tofile(f), 162

tofile() method, 168

transparent colors, 141–142

Transport Layer Security (TLS), 179

triangle class, 116–117, 129

triple quote, 44

True or False, 11, 29, 90, 155

try statement, 33

tuple packing, 52

tuples, 46–47, 133, see also lists

assignment, 52–53

built-in functions for, 53–54

deleting an element, 50

immutable, 50, 51

lists and, 62–63

in for loop, 47–49

membership operator, 49

problems, 47–52

updating an element, 51

TypeError, 10

types as dynamic, 18–19

U

union in sets, 65

unpack() function, 170

updating an element, 51

upper(), 44

usemax.py, 92

V

ValueError, 34, 97

values, 4–6

val variable, 82

variable code, 44

variable parameter lists, 83–84

variables as functions, 85–88

global, 81–83

local, 81

video files, 96

*vlist expression, 83, 84

W

walksInto, 111

Web pages, 96

while loops, 30, 31

counting, 25

while statement, 21–23

with statement, 106–107

write(), 107

writing to files, 98, 107–109

X

x-yi*yi error, 73

Z

ZeroDivisionError, 34