Last modified on 25 August 2014, at 01:26

Non-Programmer's Tutorial for Python 2.6/Print version


Authors

Josh Cogliati
User:Jrincayc, jjcogliati-jan2007 AT yahoo.com

Contributors

Front matter

All example Python source code in this tutorial is granted to the public domain. Therefore you may modify it and relicense it under any license you please. Since you are expected to learn programming, the GNU Free Documentation License would require you to keep all programs that are derived from the source code in this tutorial under that license. Since the python source code is granted to the public domain, that requirement is waived.

This tutorial was originally written in LaTeX and was available at: http://www.honors.montana.edu/~jjc/easytut/. It was moved here because the other server is going away and it was being read at least ten times a day. This document is available as LaTeX, HTML, PDF, and Postscript. Go to http://jjc.freeshell.org/easytut/ (Also could try http://web.archive.org/web/*/http://www.honors.montana.edu/~jjc/easytut/ or http://www.geocities.com/jrincayc/easytut.tar.gz ) to see all these forms. There are also versions of this in Korean, Spanish, Italian and Greek in the tar file.


The Non-Programmers' Tutorial For Python is a tutorial designed to be an introduction to the Python programming language. This guide is for someone with no programming experience.

If you have programmed in other languages I recommend using Python Tutorial for Programmers written by Guido van Rossum.

If you have any questions or comments please use the discussion pages or see Non-Programmer's Tutorial for Python 2.6/Authors for author contact information. I welcome questions and comments about this tutorial. I will try to answer any questions you have as best I can.

Thanks go to James A. Brown for writing most of the Windows install info. Thanks also to Elizabeth Cogliati for complaining enough :) about the original tutorial (that is almost unusable for a non-programmer), for proofreading, and for many ideas and comments on it. Thanks to Joe Oppegaard for writing almost all the exercises. Thanks to everyone I have missed.

Other resources

See also chapter The End for some more comments.

Intro

First things first

So, you've never programmed before. As we go through this tutorial, I will attempt to teach you how to program. There really is only one way to learn to program. You must read code and write code (as computer programs are often called). I'm going to show you lots of code. You should type in code that I show you to see what happens. Play around with it and make changes. The worst that can happen is that it won't work. When I type in code it will be formatted like this:

##Python is easy to learn
print "Hello, World!"

That's so it is easy to distinguish from the other text. If you're reading this on the web, you'll notice the code is in color -- that's just to make it stand out, and to make the different parts of the code stand out from each other. The code you enter will probably not be colored, or the colors may be different, but it won't affect the code as long as you enter it the same way as it's printed here.

If the computer prints something out it will be formatted like this:

Hello, World!

(Note that printed text goes to your screen, and does not involve paper. Before computers had screens, the output of computer programs would be printed on paper.)

If you try this program out and you get a syntax error, check and see what version of python you have. If you have python 3.0, you should be using the Non-Programmer's Tutorial for Python 3.0. This article was made for Python 2.6

There will often be a mixture of the text you type (which is shown in bold) and the text the program prints to the screen, which would look like this:

Halt!
Who Goes there? Josh
You may pass, Josh

(Some of the tutorial has not been converted to this format. Since this is a wiki, you can convert it when you find it.)

I will also introduce you to the terminology of programming - for example, that programming is often referred to as coding. This will not only help you understand what programmers are talking about, but also help the learning process.

Now, on to more important things. In order to program in Python you need the Python software. If you don't already have the Python software go to http://www.python.org/download/ and get the proper version for your platform. Download it, read the instructions and get it installed.

Installing Python

For Python programming you need a working Python installation and a text editor. Python comes with its own editor IDLE, which is quite nice and totally sufficient for the beginning. As you get more into programming, you will probably switch to some other editor like emacs, vi or another.

The Python download page is http://www.python.org/download. The most recent version is 3.1, but any Python 2.x version since 2.2 will work for this tutorial. Be careful with the upcoming Python 3, though, as some major details will change and break this tutorial's examples. A version of this tutorial for Python 3 is at Non-Programmer's Tutorial for Python 3. There are various different installation files for different computer platforms available on the download site. Here are some specific instructions for the most common operating systems:

Linux, BSD and Unix users

You are probably lucky and Python is already installed on your machine. To test it type python on a command line. If you see something like that in the following section, you are set.

If you have to install Python, just use the operating system's package manager or go to the repository where your packages are available and get Python. Alternatively, you can compile Python from scratch after downloading the source code. If you get the source code make sure you compile in the Tk extension if you want to use IDLE.

Mac users

Starting from Mac OS X (Tiger), Python ships by default with the operating system, but you might want to update to the newer version (check the version by starting python in a command line terminal). Also IDLE (the Python editor) might be missing in the standard installation. If you want to (re-)install Python, have a look at the Mac page on the Python download site.

Windows users

Some computer manufacturers pre-install Python. To check if you already have it installed, open command prompt (cmd in run menu) or MS-DOS and type python. If it says "Bad command or file name" you will need to download the appropriate Windows installer (the normal one, if you do not have a 64-bit AMD or Intel chip). Start the installer by double-clicking it and follow the procedure. Python for windows can be downloaded from the official site of python

Interactive Mode

Go into IDLE (also called the Python GUI). You should see a window that has some text like this:

Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08) [MSC v.1310 32 bit (Intel)] on win32
Type "copyright", "credits" or "license()" for more information.


   ****************************************************************
   Personal firewall software may warn about the connection IDLE
   makes to its subprocess using this computer's internal loopback
   interface.  This connection is not visible on any external
   interface and no data is sent to or received from the Internet.
   ****************************************************************


IDLE 1.2.1      
>>>

The >>> is Python's way of telling you that you are in interactive mode. In interactive mode what you type is immediately run. Try typing 1+1 in. Python will respond with 2. Interactive mode allows you to test out and see what Python will do. If you ever feel you need to play with new Python statements, go into interactive mode and try them out.

Creating and Running Programs

Go into IDLE if you are not already. In the menu at the top, select File then New Window. In the new window that appears, type the following:

print "Hello, World!"

Now save the program: select File from the menu, then Save. Save it as "hello.py" (you can save it in any folder you want). Now that it is saved it can be run.

Next run the program by going to Run then Run Module (or if you have a older version of IDLE use Edit then Run script). This will output Hello, World! on the *Python Shell* window.

For a more in-depth introduction to IDLE, a longer tutorial with screenshots can be found at http://hkn.eecs.berkeley.edu/~dyoo/python/idle_intro/index.html

Running Python Programs in Unix

If you are using Unix (such as Linux, Mac OSX, or BSD), if you make the program executable with chmod, and have as the first line:

#!/usr/bin/env python2

you can run the python program with ./hello.py like any other command.

Note: In some computer environments, you need to write:

#!/usr/bin/env python

Example for Solaris:

#!/usr/bin/python

Program file names

It is very useful to stick to some rules regarding the file names of Python programs. Otherwise some things might go wrong unexpectedly. These don't matter as much for programs, but you can have weird problems if you don't follow them for module names (modules will be discussed later).

  1. Always save the program with the extension .py. Do not put another dot somewhere else in the file name.
  2. Only use standard characters for file names: letters, numbers, dash (-) and underscore (_).
  3. White space (" ") should not be used at all (e.g. use underscores instead).
  4. Do not use anything other than a letter (particularly no numbers!) at the beginning of a file name.
  5. Do not use "non-english" characters (such as ä, ö, ü, å or ß) in your file names, or, even better, do not use them at all when programming.

Using Python from the command line

If you don't want to use Python from the command line, you don't have to, just use IDLE. To get into interactive mode just type python without any arguments. To run a program, create it with a text editor (Emacs has a good Python mode) and then run it with python program_name.

Additionally, to use Python within Vim, you may want to visit Using vim as a Python IDE

Where to get help

At some point in your Python career you will probably get stuck and have no clue about how to solve the problem you are supposed to work on. This tutorial only covers the basics of Python programming, but there is a lot of further information available.

Python documentation

First of all, Python is very well documented. There might even be copies of these documents on your computer, which came with your Python installation: * The official Python Tutorial by Guido van Rossum is often a good starting point for general questions.

  • For questions about standard modules (you will learn what this is later), the Python Library Reference is the place to look.
  • If you really want to get to know something about the details of the language, the Python Reference Manual is comprehensive but quite complex for beginners.

Python user community

There are a lot of other Python users out there, and usually they are nice and willing to help you. This very active user community is organised mostly through mailing lists and a newsgroup:

  • The tutor mailing list is for folks who want to ask questions regarding how to learn computer programming with the Python language.
  • The python-help mailing list is python.org's help desk. You can ask a group of knowledgeable volunteers questions about all your Python problems.
  • The Python newsgroup comp.lang.python (Google groups archive) is the place for general Python discussions, questions and the central meeting point of the community.

In order not to reinvent the wheel and discuss the same questions again and again, people will appreciate very much if you do a web search for a solution to your problem before contacting these lists!

Hello, World

What you should know

You should know how to edit programs in a text editor or IDLE, save the file and run the file once the files have been saved to your disk.

Printing

Programming tutorials since the beginning of time have started with a little program called "Hello, World!"[1] The syntax changed in Python 3.0. If you are using Python 3.0, you should be reading Non-Programmer's Tutorial for Python 3 instead. So here is the Python 2.6 example:

print "Hello, World!"

If you are using the command line to run programs then type it in with a text editor, save it as hello.py and run it with python hello.py

Otherwise go into IDLE, create a new window, and create the program as in section Creating and Running Programs.

When this program is run here's what it prints:

Hello, World!

Now I'm not going to tell you this every time, but when I show you a program I recommend that you type it in and run it. I learn better when I type it in and you probably do too.

Now here is a more complicated program:

print "Jack and Jill went up a hill"
print "to fetch a pail of water;"
print "Jack fell down, and broke his crown,"
print "and Jill came tumbling after."

When you run this program it prints out:

Jack and Jill went up a hill
to fetch a pail of water;
Jack fell down, and broke his crown,
and Jill came tumbling after.

When the computer runs this program it first sees the line:

print "Jack and Jill went up a hill"

so the computer prints:

Jack and Jill went up a hill

Then the computer goes down to the next line and sees:

print "to fetch a pail of water;"

So the computer prints to the screen:

to fetch a pail of water;

The computer keeps looking at each line, follows the command and then goes on to the next line. The computer keeps running commands until it reaches the end of the program.

Terminology

Now is probably a good time to give you a bit of an explanation of what is happening - and a little bit of programming terminology.

What we were doing above was using a command called print. The print command is followed by one or more arguments. So in this example

print "Hello, World!"

there is one argument, which is "Hello, World!". Note that this argument is a group of characters enclosed in double quotes ("). This is commonly referred to as a string of characters, or string, for short. Another example of a string is "Jack and Jill went up a hill".

A command and its arguments are collectively referred to as a statement, so

print "Hello, World!"

is an example of a statement.

That's probably more than enough terminology for now.

Expressions

Here is another program:

print "2 + 2 is", 2 + 2
print "3 * 4 is", 3 * 4
print "100 - 1 is", 100 - 1
print "(33 + 2) / 5 + 11.5 is", (33 + 2) / 5 + 11.5

And here is the output when the program is run:

2 + 2 is 4
3 * 4 is 12
100 - 1 is 99
(33 + 2) / 5 + 11.5 is 18.5

As you can see, Python can turn your six hundred dollar computer into a 2 dollar calculator.

In this example, the print command is followed by two arguments, with each of the arguments separated by a comma. So with the first line of the program

print "2 + 2 is", 2 + 2

The first argument is the string "2 + 2 is" and the second argument is the mathematical expression 2 + 2, which is commonly referred to as an expression.

What is important to note is that a string is printed as is (the string is what is within the double quotes but doesn't include the double quotes themselves. So the string is printed without the enclosing double quotes.) But an expression is evaluated, (in other words, converted) to its actual value.

Python has six basic operations for numbers:

Operation Symbol Example
Power (exponentiation) ** 5 ** 2 == 25
Multiplication * 2 * 3 == 6
Division / 14 / 3 == 4
Remainder (modulo) % 14 % 3 == 2
Addition + 1 + 2 == 3
Subtraction - 4 - 3 == 1

Notice that division follows the rule, if there are no decimals to start with, there will be no decimals to end with. The following program shows this:

print "14 / 3 = ", 14 / 3
print "14 % 3 = ", 14 % 3
print
print "14.0 / 3.0 =", 14.0 / 3.0
print "14.0 % 3.0 =", 14.0 % 3.0
print
print "14.0 / 3 =", 14.0 / 3
print "14.0 % 3 =", 14.0 % 3
print
print "14 / 3.0 =", 14 / 3.0
print "14 % 3.0 =", 14 % 3.0
print

With the output:

14 / 3 = 4
14 % 3 = 2

14.0 / 3.0 = 4.66666666667
14.0 % 3.0 = 2.0

14.0 / 3 = 4.66666666667
14.0 % 3 = 2.0

14 / 3.0 = 4.66666666667
14 % 3.0 = 2.0

Notice how Python gives different answers for some problems depending on whether or not decimal values are used.

The order of operations is the same as in math:

  • parentheses ()
  • exponents **
  • multiplication *, division /, and remainder %
  • addition + and subtraction -

So use parentheses to structure your formulas when needed.

Talking to humans (and other intelligent beings)

Often in programming you are doing something complicated and may not in the future remember what you did. When this happens, the program should probably be commented. A comment is a note to you and other programmers explaining what is happening. For example:

# Not quite PI, but an incredible simulation
print 22.0 / 7.0    # 355/113 is even more incredible rational approx to PI

Which outputs

3.14285714286

Notice that the comment starts with a hash: #. Comments are used to communicate with others who read the program and your future self to make clear what is complicated.

Note that any text can follow a comment, and that when the program is run, the text after the # through to the end of that line is ignored. The # does not have to be at the beginning of a new line:

# Output PI on the screen
print 22.0 / 7.0 # Well, just a good approximation

Examples

Each chapter (eventually) will contain examples of the programming features introduced in the chapter. You should at least look over them and see if you understand them. If you don't, you may want to type them in and see what happens. Mess around with them, change them and see what happens.

Denmark.py

print "Something's rotten in the state of Denmark."
print "                -- Shakespeare"

Output:

Something's rotten in the state of Denmark.
                -- Shakespeare

School.py

# This is not quite true outside of USA
# and is based on my dim memories of my younger years
print "First Grade"
print "1 + 1 =", 1 + 1
print "2 + 4 =", 2 + 4
print "5 - 2 =", 5 - 2
print
print "Third Grade"
print "243 - 23 =", 243 - 23
print "12 * 4 =", 12 * 4
print "12 / 3 =", 12 / 3
print "13 / 3 =", 13 / 3, "R", 13 % 3
print
print "Junior High"
print "123.56 - 62.12 =", 123.56 - 62.12
print "(4 + 3) * 2 =", (4 + 3) * 2
print "4 + 3 * 2 =", 4 + 3 * 2
print "3 ** 2 =", 3 ** 2
print

Output:

First Grade
1 + 1 = 2
2 + 4 = 6
5 - 2 = 3

Third Grade
243 - 23 = 220
12 * 4 = 48
12 / 3 = 4
13 / 3 = 4 R 1

Junior High
123.56 - 62.12 = 61.44
(4 + 3) * 2 = 14
4 + 3 * 2 = 10
3 ** 2 = 9

Exercises

  1. Write a program that prints your full name and your birthday as separate strings.
  2. Write a program that shows the use of all 6 math functions.


Solution

1. Write a program that prints your full name and your birthday as separate strings.

print "Ada Lovelace", "born on", "November 27, 1852"

2. Write a program that shows the use of all 6 math operations.

#Anything along these lines is acceptable:
 
#Addition
print "2 + 5 = ", 2 + 5
 
#subtraction
print "9 - 3 = ", 9 - 3
 
#multiplication
print "3 * 3 = ", 3 * 3
 
#division
print "90 / 5 = ", 90 / 5
 
#exponents
print "7 to the power of 2 (squared) = ", 7 ** 2
 
#remainder
print "the remainder when doing 22 / 9 = ", 22 % 9



Footnotes

  1. List of "Hello, world!" programs in many programming languages

Who Goes There?

Input and Variables

Now I feel it is time for a really complicated program. Here it is:

print "Halt!"
user_reply = raw_input("Who goes there? ")
print "You may pass,", user_reply

When I ran it, here is what my screen showed:

Halt!
Who goes there? Josh
You may pass, Josh

Note: After running the code by pressing F5, the Python shell will only give output:

Halt!
Who goes there?

You need to enter your name in the Python shell, and then press Enter to get the rest of the output.

Of course when you run the program your screen will look different because of the raw_input() statement. When you ran the program you probably noticed (you did run the program, right?) that you had to type in your name and then press Enter. Then the program printed out some more text and also your name. This is an example of input. The program reaches a certain point and then waits for the user to input some data that the program can use later.

Of course, getting information from the user would be useless if we didn't have anywhere to put that information and this is where variables come in. In the previous program, user_reply is a variable. Variables are like a box that can store some piece of data. Here is a program to show examples of variables:

a = 123.4
b23 = 'Spam'
first_name = "Bill"
b = 432
c = a + b
print "a + b is", c
print "first_name is", first_name
print "Sorted Parts, After Midnight or", b23

And here is the output:

a + b is 555.4
first_name is Bill
Sorted Parts, After Midnight or Spam

The variables in the above program are a, b23, first_name, b, and c. A variable in Python can store any type of data - in this example we stored some strings (e.g. "Bill") and some numbers (e.g. 432).

Note the difference between strings and variable names. Strings are marked with quotation marks, which tells the computer "don't try to understand, just take this text as it is":

print "first_name"

This would print the text:

first_name

as-is. Variable names are written without any quotation marks and instruct the computer "use the value I've previously stored under this name":

print first_name

which would print (after the previous example):

Bill

Assignment

Okay, so we have these boxes called variables and also data that can go into the variable. The computer will see a line like first_name = "Bill" and it reads it as "Put the string Bill into the box (or variable) first_name". Later on it sees the statement c = a + b and it reads it as "put the sum of a + b or 123.4 + 432 which equals 555.4 into c". The right hand side of the statement (a + b) is evaluated and the result is stored in the variable on the left hand side (c). This is called assignment, and you should not confuse the assignment equal sign (=) with "equality" in a mathematical sense here (that's what == will be used for later).

Here is another example of variable usage:

a = 1
print a
a = a + 1
print a
a = a * 2
print a

And of course here is the output:

1
2
4

Even if it is the same variable on both sides the computer still reads it as "First find out the data to store and then find out where the data goes".

One more program before I end this chapter:

number = input("Type in a number: ")
text = raw_input("Type in a string: ")
print "number =", number
print "number is a", type(number)
print "number * 2 =", number * 2
print "text =", text
print "text is a", type(text)
print "text * 2 =", text * 2

The output I got was:

Type in a Number: 12.34
Type in a String: Hello
number = 12.34
number is a <type 'float'>
number * 2 = 24.68
text = Hello
text is a <type 'str'>
text * 2 = HelloHello

Notice that number was gotten with input() while text was gotten with raw_input(). raw_input() returns a string while input() returns a number. When you want the user to type in a number use input() but if you want the user to type in a string use raw_input().

The second half of the program uses type() which tells what a variable is. Numbers are of type int or float, which are short for integer and floating point (mostly used for decimal numbers), respectively. Text strings are of type str, short for string. Integers and floats can be worked on by mathematical functions, strings cannot. Notice how when python multiplies a number by an integer the expected thing happens. However when a string is multiplied by an integer the result is that multiple copies of the string are produced (i.e., text * 2 = HelloHello).

The operations with strings do different things than operations with numbers. Here are some interactive mode examples to show that some more.

>>> "This" + " " + "is" + " joined."
'This is joined.'
>>> "Ha, " * 5
'Ha, Ha, Ha, Ha, Ha, '
>>> "Ha, " * 5 + "ha!"
'Ha, Ha, Ha, Ha, Ha, ha!'
>>> 

This could also be done as a program:

print "This" + " " + "is" + " joined."
print "Ha, " * 5
print "Ha, " * 5 + "ha!"

Here is the list of some string operations:

Operation Symbol Example
Repetition * "i" * 5 == "iiiii"
Concatenation + "Hello, " + "World!" == "Hello, World!"

Examples

Rate_times.py

# This program calculates rate and distance problems
print "Input a rate and a distance"
rate = input("Rate: ")
distance = input("Distance: ")
print "Time:", (distance / rate)

Sample runs:

Input a rate and a distance
Rate: 5
Distance: 10
Time: 2
Input a rate and a distance
Rate: 3.52
Distance: 45.6
Time: 12.9545454545

Area.py

# This program calculates the perimeter and area of a rectangle
print "Calculate information about a rectangle"
length = input("Length: ")
width = input("Width: ")
print "Area", length * width
print "Perimeter", 2 * length + 2 * width

Sample runs:

Calculate information about a rectangle
Length: 4
Width: 3
Area 12
Perimeter 14
Calculate information about a rectangle
Length: 2.53
Width: 5.2
Area 13.156
Perimeter 15.46

temperature.py

# Converts Fahrenheit to Celsius
temp = input("Fahrenheit temperature: ")
print (temp - 32.0) * 5.0 / 9.0

Sample runs:

Fahrenheit temperature: 32
0.0
Fahrenheit temperature: -40
-40.0
Fahrenheit temperature: 212
100.0
Fahrenheit temperature: 98.6
37.0

Exercises

  1. Write a program that gets 2 string variables and 2 integer variables from the user, concatenates (joins them together with no spaces) and displays the strings, then multiplies the two numbers on a new line.
Solution

Write a program that gets 2 string variables and 2 integer variables from the user, concatenates (joins them together with no spaces) and displays the strings, then multiplies the two numbers on a new line.

 
string1 = raw_input('String 1: ')
string2 = raw_input('String 2: ')
int1 = input('Integer 1: ')
int2 = input('Integer 2: ')
print string1 + string2
print int1 * int2

Another Solution

print "this is an exercise"
number_1 = input("please input the first number: ")
number_2 = input("Please input the second number: ")
 
string_1 = raw_input("Please input the first half of the word: ")
string_2 = raw_input("please input the second half of the word: ")
 
print "the words you input is '" + string_1 + string_2 + "'"
print "the result of the 2 numbers is:", number_1 * number_2


Count to 10

While loops

Here we present our first control structure. Ordinarily the computer starts with the first line and then goes down from there. Control structures change the order that statements are executed or decide if a certain statement will be run. Here's the source for a program that uses the while control structure:

a = 0
while a < 10:
    a = a + 1
    print a

And here is the extremely exciting output:

1
2
3
4
5
6
7
8
9
10

And you thought it couldn't get any worse after turning your computer into a five dollar calculator?

So what does the program do? First it sees the line a = 0 and sets a to zero. Then it sees while a < 10: and so the computer checks to see if a < 10. The first time the computer sees this statement a is zero so it is less than 10. In other words as long as a is less than ten the computer will run the tabbed in statements. This eventually makes a equal to ten (by adding one to a again and again), and the while a < 10 is not true any longer. Reaching that point the program will not run the indented lines any longer.

Always remember to put a colon ":" after the "while" statement!

Here is another example of the use of while:

a = 1
s = 0
print 'Enter Numbers to add to the sum.'
print 'Enter 0 to quit.'
while a != 0:
    print 'Current Sum:', s
    a = input('Number? ')
    s = s + a
print 'Total Sum =', s
Enter Numbers to add to the sum.
Enter 0 to quit.
Current Sum: 0
Number? 200
Current Sum: 200
Number? -15.25
Current Sum: 184.75
Number? -151.85
Current Sum: 32.9
Number? 10.00
Current Sum: 42.9
Number? 0
Total Sum = 42.9

Notice how print 'Total Sum =', s is only run at the end. The while statement only affects the lines that are indented with whitespace. The != means "does not equal" so "while a != 0:" means: "until a is zero, run the tabbed statements that follow."

Infinite loops

Now that we have while loops, it is possible to have programs that run forever. An easy way to do this is to write a program like this:

while 1 == 1:
   print "Help, I'm stuck in a loop."

The "==" operator is used to test equality of the expressions on the two sides of the operator, just as "<" was used for "less than" before (you will get a complete list of all comparison operators in the next chapter).

This program will output Help, I'm stuck in a loop. until the heat death of the universe or until you stop it, because 1 will forever be equal to 1. The way to stop it is to hit the Control (or Ctrl) button and C (the letter) at the same time. This will kill the program. (Note: sometimes you will have to hit enter after the Control-C.)

Examples

Fibonacci.py

# This program calculates the Fibonacci sequence
a = 0
b = 1
count = 0
max_count = 20
while count < max_count:
    count = count + 1
    # we need to keep track of a since we change it
    old_a = a
    old_b = b
    a = old_b
    b = old_a + old_b
    # Notice that the , at the end of a print statement keeps it
    # from switching to a new line
    print(old_a),

Output:

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181

Note the output on a single line by use of a comma at the end of the print statement.

Password.py

# Waits until a password has been entered.  Use Control-C to break out without
# the password
 
# Note that this must not be the password so that the 
# while loop runs at least once.
password = "no password"
 
# note that != means not equal
while password != "unicorn":
    password = raw_input("Password: ")
print "Welcome in"

Sample run:

Password: auo
Password: y22
Password: password
Password: open sesame
Password: unicorn
Welcome in

Exercises

Write a program that asks the user for a Login Name and password. Then when they type "lock", they need to type in their name and password to unlock the program.

Solution

Write a program that asks the user for a Login Name and password. Then when they type "lock", they need to type in their name and password to unlock the program.

name = raw_input("What is your UserName: ")
password = raw_input("What is your Password: ")
print "To lock your computer type lock."
command = ""
input1 = ""
input2 = ""
while command != "lock":
    command = raw_input("What is your command: ")
while input1 != name:
    input1 = raw_input("What is your username: ")
while input2 != password:
    input2 = raw_input("What is your password: ")
print "Welcome back to your system!"

If you would like the program to run continuously, just add a while 1 == 1: loop around the whole thing. You will have to indent the rest of the program when you add this at the top of the code, but don't worry, you don't have to do it manually for each line! Just highlight everything you want to indent and click on "Indent" under "Format" in the top bar of the python window. Note that you can use empty strings like this: "".

Another way of doing this could be:

name = raw_input('Set name: ')
password = raw_input('Set password: ')
while 1 == 1:
    nameguess=passwordguess=key=""   # multiple assignment
    while (nameguess != name) or (passwordguess != password):
        nameguess = raw_input('Name? ')
        passwordguess = raw_input('Password? ')
    print "Welcome,", name, ". Type lock to lock."
    while key != "lock":
        key = raw_input("")

Notice the or in while (name != "user") or (password != "pass"):, which we haven't yet introduced. You can probably figure out how it works.

login = "john"
password = "tucker"
logged=2
 
while logged != 0:
    while login != "Phil":    
            login = raw_input("Login : ")
    while password != "McChicken":
            password = raw_input("Password: ")
    logged = 1
 
    print "Welcome!"
    print "To leave type lock "
 
    while logged == 1:
        leave = raw_input (">> ")
        if leave == "lock":
            logged = 0
print "Goodbye!!"

This method, although a bit more crude also works. Notice it uses the as of yet un-introduced if function.


Decisions

If statement

As always I believe I should start each chapter with a warm-up typing exercise, so here is a short program to compute the absolute value of a number:

n = input("Number? ")
if n < 0:
   print "The absolute value of", n, "is", -n
else:
   print "The absolute value of", n, "is", n

Here is the output from the two times that I ran this program:

Number? -34
The absolute value of -34 is 34
Number? 1
The absolute value of 1 is 1

So what does the computer do when it sees this piece of code? First it prompts the user for a number with the statement "n = input("Number? ")". Next it reads the line "if n < 0:". If n is less than zero Python runs the line "print "The absolute value of", n, "is", -n". Otherwise it runs the line "print "The absolute value of", n, "is", n".

More formally Python looks at whether the expression n < 0 is true or false. An if statement is followed by an indented block of statements that are run when the expression is true. Optionally after the if statement is an else statement and another indented block of statements. This second block of statements is run if the expression is false.

There are a number of different tests that an expression can have. Here is a table of all of them:

operator function
< less than
<= less than or equal to
> greater than
>= greater than or equal to
== equal
!= not equal
<> another way to say not equal (old style, not recommended)

Another feature of the if command is the elif statement. It stands for else if and means if the original if statement is false but the elif part is true, then do the elif part. And if neither the if or elif expressions are true, then do what's in the else block. Here's an example:

a = 0
while a < 10:
    a = a + 1
    if a > 5:
        print a, ">", 5
    elif a <= 7:
        print a, "<=", 7
    else:
        print "Neither test was true"

and the output:

1 <= 7
2 <= 7
3 <= 7
4 <= 7
5 <= 7
6 > 5
7 > 5
8 > 5
9 > 5
10 > 5

Notice how the elif a <= 7 is only tested when the if statement fails to be true. There can be more than one elif expression, allowing multiple tests to be done in a single if statement.

Examples

# This Program Demonstrates the use of the == operator
# using numbers
print 5 == 6
# Using variables
x = 5
y = 8
print x == y

And the output

False
False

High_low.py

# Plays the guessing game higher or lower 
 
# This should actually be something that is semi random like the
# last digits of the time or something else, but that will have to
# wait till a later chapter.  (Extra Credit, modify it to be random
# after the Modules chapter)
number = 78
guess = 0
 
while guess != number: 
    guess = input("Guess a number: ")
    if guess > number:
        print "Too high"
    elif guess < number:
        print "Too low"
 
print "Just right"

Sample run:

Guess a number: 100
Too high
Guess a number: 50
Too low
Guess a number: 75
Too low
Guess a number: 87
Too high
Guess a number: 81
Too high
Guess a number: 78
Just right

even.py

# Asks for a number.
# Prints if it is even or odd
 
number = input("Tell me a number: ")
if number % 2 == 0:
    print number, "is even."
elif number % 2 == 1:
    print number, "is odd."
else:
    print number, "is very strange."

Sample runs:

Tell me a number: 3
3 is odd.
Tell me a number: 2
2 is even.
Tell me a number: 3.14159
3.14159 is very strange.

average1.py

# keeps asking for numbers until 0 is entered.
# Prints the average value.
 
count = 0
sum = 0.0
number = 1 # set to something that will not exit the while loop immediately.
 
print "Enter 0 to exit the loop"
 
while number != 0:
    number = input("Enter a number: ")
    if number != 0:
        count = count + 1
        sum = sum + number
 
print "The average was:", sum / count

Sample runs:

Enter 0 to exit the loop
Enter a number: 3
Enter a number: 5
Enter a number: 0
The average was: 4.0
Enter 0 to exit the loop
Enter a number: 1
Enter a number: 4
Enter a number: 3
Enter a number: 0
The average was: 2.66666666667

average2.py

# keeps asking for numbers until count numbers have been entered.
# Prints the average value.
 
sum = 0.0
 
print "This program will take several numbers then average them"
count = input("How many numbers would you like to average: ")
current_count = 0
 
while current_count < count:
    current_count = current_count + 1
    print "Number", current_count
    number = input("Enter a number: ")
    sum = sum + number
 
print "The average was:", sum / count

Sample runs:

This program will take several numbers then average them
How many numbers would you like to average: 2
Number 1
Enter a number: 3
Number 2
Enter a number: 5
The average was: 4.0
This program will take several numbers then average them
How many numbers would you like to average: 3
Number 1
Enter a number: 1
Number 2
Enter a number: 4
Number 3
Enter a number: 3
The average was: 2.66666666667

Exercises

  1. Modify the higher or lower program from this section to keep track of how many times the user has entered the wrong number. If it is more than 3 times, print "That must have been complicated." Note that the program does not have to quit asking for the number before it is guessed, it just has to print this after the number is guessed.
  2. Write a program that asks for two numbers. If the sum of the numbers is greater than 100, print "That is a big number."
  3. Write a program that asks the user their name, if they enter your name say "That is a nice name", if they enter "John Cleese" or "Michael Palin", tell them how you feel about them ;), otherwise tell them "You have a nice name."
Solution

Modify the higher or lower program from this section to keep track of how many times the user has entered the wrong number. If it is more than 3 times, print "That must have been complicated."

number = 42
guess = 0
count = 0
while guess != number:
    count = count + 1
    guess = input('Guess a number: ')
    if guess > number:
        print 'Too high'
    elif guess < number:
        print 'Too low'
    else:
        print 'Just right'
        break
    if count > 2:
        print 'That must have been complicated.'
        break

Write a program that asks for two numbers. If the sum of the numbers is greater than 100, print "That is a big number."

number1 = input('1st number: ')
number2 = input('2nd number: ')
if number1 + number2 > 100:
    print 'That is a big number.'

Write a program that asks the user their name, if they enter your name say "That is a nice name", if they enter "John Cleese" or "Michael Palin", tell them how you feel about them ;), otherwise tell them "You have a nice name."

name = raw_input('Your name: ')
if name == 'Ada':
    print 'That is a nice name.'
elif name == 'John Cleese' or name == 'Michael Palin':
    print '... some funny text.'
else:
    print 'You have a nice name.'


Debugging

What is debugging?

"As soon as we started programming, we found to our surprise that it wasn't as easy to get programs right as we had thought. Debugging had to be discovered. I can remember the exact instant when I realized that a large part of my life from then on was going to be spent in finding mistakes in my own programs." — Maurice Wilkes discovers debugging, 1949

By now if you have been messing around with the programs you have probably found that sometimes the program does something you didn't want it to do. This is fairly common. Debugging is the process of figuring out what the computer is doing and then getting it to do what you want it to do. This can be tricky. I once spent nearly a week tracking down and fixing a bug that was caused by someone putting an x where a y should have been.

This chapter will be more abstract than previous chapters.

What should the program do?

The first thing to do (this sounds obvious) is to figure out what the program should be doing if it is running correctly. Come up with some test cases and see what happens. For example, let's say I have a program to compute the perimeter of a rectangle (the sum of the length of all the edges). I have the following test cases:

height width perimeter
3 4 14
2 3 10
4 4 16
2 2 8
5 1 12

I now run my program on all of the test cases and see if the program does what I expect it to do. If it doesn't then I need to find out what the computer is doing.

More commonly some of the test cases will work and some will not. If that is the case you should try and figure out what the working ones have in common. For example here is the output for a perimeter program (you get to see the code in a minute):

Height: 3
Width: 4
perimeter = 15
Height: 2
Width: 3
perimeter = 11
Height: 4
Width: 4
perimeter = 16
Height: 2
Width: 2
perimeter = 8
Height: 5
Width: 1
perimeter = 8

Notice that it didn't work for the first two inputs, it worked for the next two and it didn't work on the last one. Try and figure out what is in common with the working ones. Once you have some idea what the problem is finding the cause is easier. With your own programs you should try more test cases if you need them.

What does the program do?

The next thing to do is to look at the source code. One of the most important things to do while programming is reading source code. The primary way to do this is code walkthroughs.

A code walkthrough starts at the first line, and works its way down until the program is done. While loops and if statements mean that some lines may never be run and some lines are run many times. At each line you figure out what Python has done.

Lets start with the simple perimeter program. Don't type it in, you are going to read it, not run it. The source code is:

height = input("Height: ")
width = input("Width: ")
print "perimeter =", width + height + width + width
Question: What is the first line Python runs?
Answer: The first line is always run first. In this case it is: height = input("Height: ")
What does that line do?
Prints Height: , waits for the user to type a number in, and puts that in the variable height.
What is the next line that runs?
In general, it is the next line down which is: width = input("Width: ")
What does that line do?
Prints Width: , waits for the user to type a number in, and puts what the user types in the variable width.
What is the next line that runs?
When the next line is not indented more or less than the current line, it is the line right afterwards, so it is: print "perimeter = ", width + height + width + width (It may also run a function in the current line, but that's a future chapter.) What does that line do?
First it prints perimeter = , then it prints width + height + width + width.
Does width + height + width + width calculate the perimeter properly?
Let's see, perimeter of a rectangle is the bottom (width) plus the left side (height) plus the top (width) plus the right side (huh?). The last item should be the right side's length, or the height.
Do you understand why some of the times the perimeter was calculated "correctly"?  
It was calculated correctly when the width and the height were equal.

The next program we will do a code walkthrough for is a program that is supposed to print out 5 dots on the screen. However, this is what the program is outputting:

. . . . 

And here is the program:

number = 5
while number > 1:
    print ".",
    number = number - 1
print

This program will be more complex to walkthrough since it now has indented portions (or control structures). Let us begin.

What is the first line to be run?
The first line of the file: number = 5
What does it do?
Puts the number 5 in the variable number.
What is the next line?
The next line is: while number > 1:
What does it do?
Well, while statements in general look at their expression, and if it is true they do the next indented block of code, otherwise they skip the next indented block of code.
So what does it do right now?
If number > 1 is true then the next two lines will be run.
So is number > 1?
The last value put into number was 5 and 5 > 1 so yes.
So what is the next line?
Since the while was true the next line is: print ".",
What does that line do?
Prints one dot and since the statement ends with a ',' the next print statement will not be on a different screen line.
What is the next line?
number = number - 1 since that is following line and there are no indent changes.
What does it do?
It calculates number - 1, which is the current value of number (or 5) subtracts 1 from it, and makes that the new value of number. So basically it changes number's value from 5 to 4.
What is the next line?
Well, the indent level decreases so we have to look at what type of control structure it is. It is a while loop, so we have to go back to the while clause which is while number > 1:
What does it do?
It looks at the value of number, which is 4, and compares it to 1 and since 4 > 1 the while loop continues.
What is the next line?
Since the while loop was true, the next line is: print ".",
What does it do?
It prints a second dot on the line.
What is the next line?
No indent change so it is: number = number - 1
And what does it do?
It takes the current value of number (4), subtracts 1 from it, which gives it 3 and then finally makes 3 the new value of number.
What is the next line?
Since there is an indent change caused by the end of the while loop, the next line is: while number > 1:
What does it do?
It compares the current value of number (3) to 1. 3 > 1 so the while loop continues.
What is the next line?
Since the while loop condition was true the next line is: print ".",
And it does what?
A third dot is printed on the line.
What is the next line?
It is: number = number - 1
What does it do?
It takes the current value of number (3) subtracts from it 1 and makes the 2 the new value of number.
What is the next line?
Back up to the start of the while loop: while number > 1:
What does it do?
It compares the current value of number (2) to 1. Since 2 > 1 the while loop continues.
What is the next line?
Since the while loop is continuing: print ".",
What does it do?
It discovers the meaning of life, the universe and everything. I'm joking. (I had to make sure you were awake.) The line prints a fourth dot on the screen.
What is the next line?
It's: number = number - 1
What does it do?
Takes the current value of number (2) subtracts 1 and makes 1 the new value of number.
What is the next line?
Back up to the while loop: while number > 1:
What does the line do?
It compares the current value of number (1) to 1. Since 1 > 1 is false (one is not greater than one), the while loop exits.
What is the next line?
Since the while loop condition was false the next line is the line after the while loop exits, or: print
What does that line do?
Makes the screen go to the next line.
Why doesn't the program print 5 dots?
The loop exits 1 dot too soon.
How can we fix that?
Make the loop exit 1 dot later.
And how do we do that?
There are several ways. One way would be to change the while loop to: while number > 0: Another way would be to change the conditional to: number >= 1 There are a couple others.

How do I fix the program?

You need to figure out what the program is doing. You need to figure out what the program should do. Figure out what the difference between the two is. Debugging is a skill that has to be practiced to be learned. If you can't figure it out after an hour, take a break, talk to someone about the problem or contemplate the lint in your navel. Come back in a while and you will probably have new ideas about the problem. Good luck.


Defining Functions

Creating Functions

To start off this chapter I am going to give you an example of what you could do but shouldn't (so don't type it in):

a = 23
b = -23
 
if a < 0:
    a = -a
if b < 0:
    b = -b
if a == b:
    print "The absolute values of", a, "and", b, "are equal"
else:
    print "The absolute values of", a, "and", b, "are different"

with the output being:

The absolute values of 23 and 23 are equal

The program seems a little repetitive. Programmers hate to repeat things -- that's what computers are for, after all! (Note also that finding the absolute value changed the value of the variable, which is why it is printing out 23, and not -23 in the output.) Fortunately Python allows you to create functions to remove duplication. Here is the rewritten example:

def absolute_value(n):
    if n < 0:
        n = -n
    return n
 
a = 23
b = -23
 
if absolute_value(a) == absolute_value(b):
    print "The absolute values of", a, "and", b, "are equal"
else:
    print "The absolute values of", a, "and", b, "are different"

with the output being:

The absolute values of 23 and -23 are equal

The key feature of this program is the def statement. def (short for define) starts a function definition. def is followed by the name of the function absolute_value. Next comes a '(' followed by the parameter n (n is passed from the program into the function when the function is called). The statements after the ':' are executed when the function is used. The statements continue until either the indented statements end or a return is encountered. The return statement returns a value back to the place where the function was called.

Notice how the values of a and b are not changed. Functions can be used to repeat tasks that don't return values. Here are some examples:

def hello():
    print "Hello"
 
def area(w, h):
    return w * h
 
def print_welcome(name):
    print "Welcome", name
 
hello()
hello()
 
print_welcome("Fred")
w = 4
h = 5
print "width =", w, "height =", h, "area =", area(w, h)

with output being:

Hello
Hello
Welcome Fred
width = 4 height = 5 area = 20

That example shows some more stuff that you can do with functions. Notice that you can use no arguments or two or more. Notice also when a function doesn't need to send back a value, a return is optional.

Variables in functions

When eliminating repeated code, you often have variables in the repeated code. In Python, these are dealt with in a special way. So far all variables we have seen are global variables. Functions have a special type of variable called local variables. These variables only exist while the function is running. When a local variable has the same name as another variable (such as a global variable), the local variable hides the other. Sound confusing? Well, these next examples (which are a bit contrived) should help clear things up.

a = 4
 
def print_func():
    a = 17
    print "in  print_func a = ", a
 
print_func()
print "a = ", a,"which is global variable assigned prior to the function print_func"

When run, we will receive an output of:

in print_func a = 17
a = 4 which is global variable assigned prior to the function print_func

Variable assignments inside a function do not override global variables, they exist only inside the function. Even though a was assigned a new value inside the function, this newly assigned value was only relevant to print_func, when the function finishes running, and the a's values is printed again, we see the originally assigned values.

Complex example

a_var = 10
b_var = 15
c_var = 25
 
def a_func(a_var):
    print "in a_func a_var = ", a_var
    b_var = 100 + a_var
    d_var = 2 * a_var
    print "in a_func b_var = ", b_var
    print "in a_func d_var = ", d_var
    print "in a_func c_var = ", c_var
    return b_var + 10
 
c_var = a_func(b_var)
 
print "a_var = ", a_var
print "b_var = ", b_var
print "c_var = ", c_var
print "d_var = ", d_var

The output is:

 in a_func a_var =  15
 in a_func b_var =  115
 in a_func d_var =  30
 in a_func e_var =  25
 a_var =  10
 b_var =  15
 c_var =  125
 d_var = 
 
 Traceback (most recent call last):
  File "C:\Python24\def2", line 19, in -toplevel-
     print "d_var = ", d_var
 
 NameError: name 'd_var' is not defined

In this example the variables a_var, b_var, and d_var are all local variables when they are inside the function a_func. After the statement return b_var + 10 is run, they all cease to exist. The variable a_var is automatically a local variable since it is a parameter name. The variables b_var and d_var are local variables since they appear on the left of an equals sign in the function in the statements b_var = 100 + a_var and d_var = 2 * a_var .

Inside of the function a_var has no value assigned to it. When the function is called with c_var = a_func(b_var), 15 is assigned to a_var since at that point in time b_var is 15, making the call to the function a_func(15). This ends up setting a_var to 15 when it is inside of a_func.

As you can see, once the function finishes running, the local variables a_var and b_var that had hidden the global variables of the same name are gone. Then the statement print "a_var = ", a_var prints the value 10 rather than the value 15 since the local variable that hid the global variable is gone.

Another thing to notice is the NameError that happens at the end. This appears since the variable d_var no longer exists since a_func finished. All the local variables are deleted when the function exits. If you want to get something from a function, then you will have to use return something.

One last thing to notice is that the value of e_var remains unchanged inside a_func since it is not a parameter and it never appears on the left of an equals sign inside of the function a_func. When a global variable is accessed inside a function it is the global variable from the outside.

Functions allow local variables that exist only inside the function and can hide other variables that are outside the function.

Examples

temperature2.py

# converts temperature to fahrenheit or celsius
 
def print_options():
    print "Options:"
    print " 'p' print options"
    print " 'c' convert from celsius"
    print " 'f' convert from fahrenheit"
    print " 'q' quit the program"
 
def celsius_to_fahrenheit(c_temp):
    return 9.0 / 5.0 * c_temp + 32
 
def fahrenheit_to_celsius(f_temp):
    return (f_temp - 32.0) * 5.0 / 9.0
 
choice = "p"
while choice != "q":
    if choice == "c":
        temp = input("Celsius temperature: ")
        print "Fahrenheit:", celsius_to_fahrenheit(temp)
    elif choice == "f":
        temp = input("Fahrenheit temperature: ")
        print "Celsius:", fahrenheit_to_celsius(temp)
    elif choice != "p":
        print_options()
    choice = raw_input("option: ")

Sample Run:

Options:
 'p' print options
 'c' convert from celsius
 'f' convert from fahrenheit
 'q' quit the program
option: c
Celsius temperature: 30 
Fahrenheit: 86.0
option: f
Fahrenheit temperature: 60
Celsius: 15.5555555556
option: q

area2.py

# By Amos Satterlee
print
def hello():
    print 'Hello!'
 
def area(width, height):
    return width * height
 
def print_welcome(name):
    print 'Welcome,', name
 
name = raw_input('Your Name: ')
hello(),
print_welcome(name)
print
print 'To find the area of a rectangle,'
print 'enter the width and height below.'
print
w = input('Width: ')
while w <= 0:
    print 'Must be a positive number'
    w = input('Width: ')
 
h = input('Height: ')
while h <= 0:
    print 'Must be a positive number'
    h = input('Height: ')
 
print 'Width =', w, 'Height =', h, 'so Area =', area(w, h)

Sample Run:

Your Name: Josh
Hello!
Welcome, Josh

To find the area of a rectangle,
enter the width and height below.

Width: -4
Must be a positive number
Width: 4
Height: 3
Width = 4 Height = 3 so Area = 12

Exercises

Rewrite the area2.py program from the Examples above to have a separate function for the area of a square, the area of a rectangle, and the area of a circle (3.14 * radius ** 2). This program should include a menu interface.

Solution

Rewrite the area2.py program from the Examples above to have a separate function for the area of a square, the area of a rectangle, and the area of a circle (3.14 * radius ** 2). This program should include a menu interface.

def square(length):
    return length * length
 
def rectangle(width , height):
    return width * height
 
def circle(radius):
    return 3.14 * radius ** 2
 
def options():
    print
    print "Options:"
    print "s = calculate the area of a square."
    print "c = calculate the area of a circle."
    print "r = calculate the area of a rectangle."
    print "q = quit"
    print
 
print "This program will calculate the area of a square, circle or rectangle."
choice = "x"
options()
while choice != "q":
    choice = raw_input("Please enter your choice: ")
    if choice == "s":
        length = input("Length of square: ")
        print "The area of this square is", square(length)
        options()
    elif choice == "c":
        radius = input("Radius of the circle: ")
        print "The area of the circle is", circle(radius)
        options()
    elif choice == "r":
        width = input("Width of the rectangle: ")
        height = input("Height of the rectangle: ")
        print "The area of the rectangle is", rectangle(width, height)
        options()
    elif choice == "q":
        print "",
    else:
        print "Unrecognized option."
        options()


Advanced Functions Example

Some people find this section useful, and some find it confusing. If you find it confusing you can skip it (or just look at the examples.) Now we will do a walk through for the following program:

def mult(a, b):
    if b == 0:
        return 0
    rest = mult(a, b - 1)
    value = a + rest
    return value
print "3 * 2 = ", mult(3, 2)
Output
3 * 2 =  6

Basically this program creates a positive integer multiplication function (that is far slower than the built in multiplication function) and then demonstrates this function with a use of the function. This program demonstrates the use of recursion, that is a form of iteration (repetition) in which there is a function that repeatedly calls itself until an exit condition is satisfied. It uses repeated additions to give the same result as mutiplication: e.g. 3 + 3 (addition) gives the same result as 3 * 2 (multiplication).


RUN 1

Question: What is the first thing the program does?
Answer: The first thing done is the function mult is defined with all the lines except the last one.
function mult defined
def mult(a, b):
    if b == 0:
        return 0
    rest = mult(a, b - 1)
    value = a + rest
    return value
This creates a function that takes two parameters and returns a value when it is done. Later this function can be run.

What happens next?
The next line after the function, print "3 * 2 = ", mult(3, 2) is run.

And what does this do?
It prints 3 * 2 = and the return value of mult(3, 2)

And what does mult(3, 2) return?
We need to do a walkthrough of the mult function to find out.

RUN 2

What happens next?
The variable a gets the value 3 assigned to it and the variable b gets the value 2 assigned to it.

And then?
The line if b == 0: is run. Since b has the value 2 this is false so the line return 0 is skipped.

And what then?
The line rest = mult(a, b - 1) is run. This line sets the local variable rest to the value of mult(a, b - 1). The value of a is 3 and the value of b is 2 so the function call is mult(3,1)

So what is the value of mult(3, 1) ?
We will need to run the function mult with the parameters 3 and 1.
RUN 2
def mult(3, 2):
    if b == 0:
        return 0
    rest = mult(3, 2 - 1)
    rest = mult(3, 1)
    value = 3 + rest
    return value



RUN 3

So what happens next?
The local variables in the new run of the function are set so that a has the value 3 and b has the value 1. Since these are local values these do not affect the previous values of a and b.

And then?
Since b has the value 1 the if statement is false, so the next line becomes rest = mult(a, b - 1).

What does this line do?
This line will assign the value of mult(3, 0) to rest.

So what is that value?
We will have to run the function one more time to find that out. This time a has the value 3 and b has the value 0.

So what happens next?
The first line in the function to run is if b == 0:. b has the value 0 so the next line to run is return 0

And what does the line return 0 do?
This line returns the value 0 out of the function.

So?
So now we know that mult(3, 0) has the value 0. Now we know what the line rest = mult(a, b - 1) did since we have run the function mult with the parameters 3 and 0. We have finished running mult(3, 0) and are now back to running mult(3, 1). The variable rest gets assigned the value 0.

What line is run next?
The line value = a + rest is run next. In this run of the function, a = 3 and rest = 0 so now value = 3.

What happens next?
The line return value is run. This returns 3 from the function. This also exits from the run of the function mult(3, 1). After return is called, we go back to running mult(3, 2).

Where were we in mult(3, 2)?
We had the variables a = 3 and b = 2 and were examining the line rest = mult(a, b - 1).

So what happens now?
The variable rest get 3 assigned to it. The next line value = a + rest sets value to 3 + 3 or 6.

So now what happens?
The next line runs, this returns 6 from the function. We are now back to running the line print "3 * 2 = ", mult(3, 2) which can now print out the 6.

What is happening overall?
Basically we used two facts to calculate the multiple of the two numbers. The first is that any number times 0 is 0 (x * 0 = 0). The second is that a number times another number is equal to the first number plus the first number times one less than the second number (x * y = x + x * (y - 1)). So what happens is 3 * 2 is first converted into 3 + 3 * 1. Then 3 * 1 is converted into 3 + 3 * 0. Then we know that any number times 0 is 0 so 3 * 0 is 0. Then we can calculate that 3 + 3 * 0 is 3 + 0 which is 3. Now we know what 3 * 1 is so we can calculate that 3 + 3 * 1 is 3 + 3 which is 6.

This is how the whole thing works:

3 * 2
3 + 3 * 1
3 + 3 + 3 * 0
3 + 3 + 0
3 + 3
6

Should you still have problems with this example, look at the process backwards. What is the last step that happens? We can easily make out that the result of mult(3, 0) is 0. Since b is 0, the function mult(3, 0) will return 0 and stop.

So what does the previous step do? mult(3, 1) does not return 0 because b is not 0. So the next lines are executed: rest = mult (a, b - 1), which is rest = mult (3, 0), which is 0 as we just worked out. So now the variable rest is set to 0.

The next line adds the value of rest to a, and since a is 3 and rest is 0, the result is 3.

Now we know that the function mult(3, 1) returns 3. But we want to know the result of mult(3,2). Therefore, we need to jump back to the start of the program and execute it one more round: mult(3, 2) sets rest to the result of mult(3, 1). We know from the last round that this result is 3. Then value calculates as a + rest, i. e. 3 + 3. Then the result of 3 * 2 is printed as 6.

The point of this example is that the function mult(a, b) starts itself inside itself. It does this until b reaches 0 and then calculates the result as explained above.

Recursion

Programming constructs of this kind are called recursive and probably the most intuitive definition of recursion is:


Recursion
If you still don't get it, see recursion.

These last two sections were recently written. If you have any comments, found any errors or think I need more/clearer explanations please email. I have been known in the past to make simple things incomprehensible. If the rest of the tutorial has made sense, but this section didn't, it is probably my fault and I would like to know. Thanks.

Examples

factorial.py

#defines a function that calculates the factorial
 
def factorial(n):
    if n <= 1:
        return 1
    return n * factorial(n - 1)
 
print "2! =", factorial(2)
print "3! =", factorial(3)
print "4! =", factorial(4)
print "5! =", factorial(5)

Output:

2! = 2
3! = 6
4! = 24
5! = 120

countdown.py

def count_down(n):
    print n
    if n > 0:
        return count_down(n-1)
 
count_down(5)

Output:

5
4
3
2
1
0

Commented mult.py

# The comments below have been numbered as steps, to make explanation
# of the code easier. Please read according to those steps.
# (step number 1, for example, is at the bottom)
 
 
def mult(a, b): # (2.) This function will keep repeating itself, because....
    if b == 0:
        return 0
    rest = mult(a, b - 1) # (3.) ....Once it reaches THIS, the sequence starts over again and goes back to the top!
    value = a + rest
    return value # (4.) therefore, "return value" will not happen until the program gets past step 3 above
 
print "3 * 2 = ", mult(3, 2) # (1.) The "mult" function will first initiate here
 
 
# The "return value" event at the end can therefore only happen
# once b equals zero (b decreases by 1 everytime step 3 happens).
# And only then can the print command at the bottom be displayed.
 
# See it as kind of a "jump-around" effect. Basically, all you
# should really understand is that the function is reinitiated
# WITHIN ITSELF at step 3. Therefore, the sequence "jumps" back
# to the top.

Commented factorial.py

# Another "jump-around" function example:
 
def factorial(n): # (2.) So once again, this function will REPEAT itself....
    if n <= 1:
        return 1
    return n * factorial(n - 1) # (3.) Because it RE-initiates HERE, and goes back to the top.
 
print "2! =", factorial(2) # (1.) The "factorial" function is initiated with this line
print "3! =", factorial(3)
print "4! =", factorial(4)
print "5! =", factorial(5)

Commented countdown.py

# Another "jump-around", nice and easy:
 
 
def count_down(n): # (2.) Once again, this sequence will repeat itself....
    print n
    if n > 0:
        return count_down(n-1) # (3.) Because it restarts here, and goes back to the top
 
count_down(5) # (1.) The "count_down" function initiates here


Lists

Variables with more than one value

You have already seen ordinary variables that store a single value. However other variable types can hold more than one value. The simplest type is called a list. Here is an example of a list being used:

which_one = input("What month (1-12)? ")
months = ['January', 'February', 'March', 'April', 'May', 'June', 'July',
          'August', 'September', 'October', 'November', 'December']
 
if 1 <= which_one <= 12:
    print "The month is", months[which_one - 1]

and an output example:

What month (1-12)? 3
The month is March

In this example the months is a list. months is defined with the lines months = ['January', 'February', 'March', 'April', 'May', 'June', 'July', and 'August', 'September', 'October', 'November', 'December'] (note that a \ could also be used to split a long line, but that is not necessary in this case because Python is intelligent enough to recognize that everything within brackets belongs together). The [ and ] start and end the list with commas (,) separating the list items. The list is used in months[which_one - 1]. A list consists of items that are numbered starting at 0. In other words if you wanted January you would use months[0]. Give a list a number and it will return the value that is stored at that location.

The statement if 1 <= which_one <= 12: will only be true if which_one is between one and twelve inclusive (in other words it is what you would expect if you have seen that in algebra).

Lists can be thought of as a series of boxes. Each box has a different value. For example, the boxes created by demolist = ['life', 42, 'the universe', 6, 'and', 9] would look like this:

box number 0 1 2 3 4 5
demolist "life" 42 "the universe" 6 "and" 9

Each box is referenced by its number so the statement demolist[0] would get 'life', demolist[1] would get 42 and so on up to demolist[5] getting 9.

More features of lists

The next example is just to show a lot of other stuff lists can do (for once I don't expect you to type it in, but you should probably play around with lists until you are comfortable with them.). Here goes:

demolist = ["life", 42, "the universe", 6, "and", 9]
print "demolist = ",demolist
demolist.append("everything")
print "after 'everything' was appended demolist is now:"
print demolist
print "len(demolist) =", len(demolist)
print "demolist.index(42) =", demolist.index(42)
print "demolist[1] =", demolist[1]
 
# Next we will loop through the list
c = 0
while c < len(demolist):
    print "demolist[", c, "] =", demolist[c]
    c = c + 1
 
del demolist[2]
print "After 'the universe' was removed demolist is now:"
print demolist
if "life" in demolist:
    print "'life' was found in demolist"
else:
    print "'life' was not found in demolist"
 
if "amoeba" in demolist:
    print "'amoeba' was found in demolist"
 
if "amoeba" not in demolist:
    print "'amoeba' was not found in demolist"
 
demolist.sort()
print "The sorted demolist is", demolist

The output is:

demolist =  ['life', 42, 'the universe', 6, 'and', 9]
after 'everything' was appended demolist is now:
['life', 42, 'the universe', 6, 'and', 9, 'everything']
len(demolist) = 7
demolist.index(42) = 1
demolist[1] = 42
demolist[ 0 ] = life
demolist[ 1 ] = 42
demolist[ 2 ] = the universe
demolist[ 3 ] = 6
demolist[ 4 ] = and
demolist[ 5 ] = 9
demolist[ 6 ] = everything
After 'the universe' was removed demolist is now:
['life', 42, 6, 'and', 9, 'everything']
'life' was found in demolist
'amoeba' was not found in demolist
The sorted demolist is [6, 9, 42, 'and', 'everything', 'life']

This example uses a whole bunch of new functions. Notice that you can just print a whole list. Next the append function is used to add a new item to the end of the list. len returns how many items are in a list. The valid indexes (as in numbers that can be used inside of the []) of a list range from 0 to len - 1. The index function tells where the first location of an item is located in a list. Notice how demolist.index(42) returns 1, and when demolist[1] is run it returns 42. The line # Next we will loop through the list is a just a reminder to the programmer (also called a comment). Python will ignore any lines that start with a #. Next the lines:

c = 0
while c < len(demolist):
    print 'demolist[', c, '] =', demolist[c]
    c = c + 1

create a variable c, which starts at 0 and is incremented until it reaches the last index of the list. Meanwhile the print statement prints out each element of the list. The del command can be used to remove a given element in a list. The next few lines use the in operator to test if an element is in or is not in a list. The sort function sorts the list. This is useful if you need a list in order from smallest number to largest or alphabetical. Note that this rearranges the list. In summary, for a list, the following operations occur:

example explanation
demolist[2] accesses the element at index 2
demolist[2] = 3 sets the element at index 2 to be 3
del demolist[2] removes the element at index 2
len(demolist) returns the length of demolist
"value" in demolist is True if "value" is an element in demolist
"value" not in demolist is True if "value" is not an element in demolist
demolist.sort() sorts demolist
demolist.index("value") returns the index of the first place that "value" occurs
demolist.append("value") adds an element "value" at the end of the list
demolist.remove("value") removes the first occurrence of value from demolist (same as del demolist[demolist.index("value")])

This next example uses these features in a more useful way:

menu_item = 0
namelist = []
while menu_item != 9:
    print "--------------------"
    print "1. Print the list"
    print "2. Add a name to the list"
    print "3. Remove a name from the list"
    print "4. Change an item in the list"
    print "9. Quit"
    menu_item = input("Pick an item from the menu: ")
    if menu_item == 1:
        current = 0
        if len(namelist) > 0:
            while current < len(namelist):
                print current, ".", namelist[current]
                current = current + 1
        else:
            print "List is empty"
    elif menu_item == 2:
        name = raw_input("Type in a name to add: ")
        namelist.append(name)
    elif menu_item == 3:
        del_name = raw_input("What name would you like to remove: ")
        if del_name in namelist:
            # namelist.remove(del_name) would work just as fine
            item_number = namelist.index(del_name)
            del namelist[item_number]
            # The code above only removes the first occurrence of
            # the name.  The code below from Gerald removes all.
            # while del_name in namelist:
            #       item_number = namelist.index(del_name)
            #       del namelist[item_number]
        else:
            print del_name, "was not found"
    elif menu_item == 4:
        old_name = raw_input("What name would you like to change: ")
        if old_name in namelist:
            item_number = namelist.index(old_name)
            new_name = raw_input("What is the new name: ")
            namelist[item_number] = new_name
        else:
            print old_name, "was not found"
 
print "Goodbye"

And here is part of the output:

--------------------
1. Print the list
2. Add a name to the list
3. Remove a name from the list
4. Change an item in the list
9. Quit

Pick an item from the menu: 2
Type in a name to add: Jack

Pick an item from the menu: 2
Type in a name to add: Jill

Pick an item from the menu: 1
0 . Jack
1 . Jill

Pick an item from the menu: 3
What name would you like to remove: Jack

Pick an item from the menu: 4
What name would you like to change: Jill
What is the new name: Jill Peters

Pick an item from the menu: 1
0 . Jill Peters

Pick an item from the menu: 9
Goodbye

That was a long program. Let's take a look at the source code. The line namelist = [] makes the variable namelist a list with no items (or elements). The next important line is while menu_item != 9:. This line starts a loop that allows the menu system for this program. The next few lines display a menu and decide which part of the program to run.

The section

current = 0
if len(namelist) > 0:
    while current < len(namelist):
        print current, ".", namelist[current]
        current = current + 1
else:
    print "List is empty"

goes through the list and prints each name. len(namelist) tells how many items are in the list. If len returns 0, then the list is empty.

Then, a few lines later, the statement namelist.append(name) appears. It uses the append function to add an item to the end of the list. Jump down another two lines, and notice this section of code:

item_number = namelist.index(del_name)
del namelist[item_number]

Here the index function is used to find the index value that will be used later to remove the item. del namelist[item_number] is used to remove a element of the list.

The next section

old_name = raw_input("What name would you like to change: ")
if old_name in namelist:
    item_number = namelist.index(old_name)
    new_name = raw_input("What is the new name: ")
    namelist[item_number] = new_name
else:
   print old_name, "was not found"

uses index to find the item_number and then puts new_name where the old_name was.

Congratulations, with lists under your belt, you now know enough of the language that you could do any computations that a computer can do (this is technically known as Turing-Completeness). Of course, there are still many features that are used to make your life easier.

Examples

test.py

## This program runs a test of knowledge
 
# First get the test questions
# Later this will be modified to use file io.
def get_questions():
    # notice how the data is stored as a list of lists
    return [["What color is the daytime sky on a clear day? ", "blue"],
            ["What is the answer to life, the universe and everything? ", "42"],
            ["What is a three letter word for mouse trap? ", "cat"]]
 
# This will test a single question
# it takes a single question in
# it returns True if the user typed the correct answer, otherwise False
 
def check_question(question_and_answer):
    # extract the question and the answer from the list
    question = question_and_answer[0]
    answer = question_and_answer[1]
    # give the question to the user
    given_answer = raw_input(question)
    # compare the user's answer to the testers answer
    if answer == given_answer:
        print "Correct"
        return True
    else:
        print "Incorrect, correct was:", answer
        return False
 
# This will run through all the questions
def run_test(questions):
    if len(questions) == 0:
        print "No questions were given."
        # the return exits the function
        return
    index = 0
    right = 0
    while index < len(questions):
        # Check the question
        if check_question(questions[index]):
            right = right + 1
            index = index + 1
        # go to the next question
        else:
            index = index + 1
    # notice the order of the computation, first multiply, then divide
    print "You got", right * 100 / len(questions),\
           "% right out of", len(questions)
 
# now let's run the questions
 
run_test(get_questions())

The values True and False point to 1 and 0, respectively. They are often used in sanity checks, loop conditions etc. You will learn more about this a little bit later (chapter Boolean Expressions).

Sample Output:

What color is the daytime sky on a clear day?green
Incorrect, correct was: blue
What is the answer to life, the universe and everything?42
Correct
What is a three letter word for mouse trap?cat
Correct
You got 66 % right out of 3

Exercises

Expand the test.py program so it has a menu giving the option of taking the test, viewing the list of questions and answers, and an option to quit. Also, add a new question to ask, "What noise does a truly advanced machine make?" with the answer of "ping".

Solution

Expand the test.py program so it has menu giving the option of taking the test, viewing the list of questions and answers, and an option to quit. Also, add a new question to ask, "What noise does a truly advanced machine make?" with the answer of "ping".

## This program runs a test of knowledge
 
questions = [["What color is the daytime sky on a clear day? ", "blue"],
             ["What is the answer to life, the universe and everything? ", "42"],
             ["What is a three letter word for mouse trap? ", "cat"],
             ["What noise does a truly advanced machine make?", "ping"]]
 
# This will test a single question
# it takes a single question in
# it returns True if the user typed the correct answer, otherwise False
 
def check_question(question_and_answer):
    # extract the question and the answer from the list
    question = question_and_answer[0]
    answer = question_and_answer[1]
    # give the question to the user
    given_answer = raw_input(question)
    # compare the user's answer to the testers answer
    if answer == given_answer:
        print "Correct"
        return True
    else:
        print "Incorrect, correct was:", answer
        return False
 
# This will run through all the questions
 
def run_test(questions):
 
    if len(questions) == 0:
        print "No questions were given."
        # the return exits the function
        return
    index = 0
    right = 0
    while index < len(questions):
        # Check the question
        if check_question(questions[index]):
            right = right + 1
        # go to the next question
        index = index + 1
    # notice the order of the computation, first multiply, then divide
    print ("You got", right * 100 / len(questions),
           "% right out of", len(questions))
 
#showing a list of questions and answers
def showquestions(questions):
    q = 0
    while q < len(questions):
        a = 0
        print "Q:" , questions[q][a]
        a = 1
        print "A:" , questions[q][a]
        q = q + 1
 
# now let's define the menu function
def menu():
    print "-----------------"
    print "Menu:"
    print "1 - Take the test"
    print "2 - View a list of questions and answers"
    print "3 - View the menu"
    print "5 - Quit"
    print "-----------------"
 
choice = "3"
while choice != "5":
    if choice == "1":
        run_test(questions)
    elif choice == "2":
        showquestions(questions)
    elif choice == "3":
        menu()
    print
    choice = raw_input("Choose your option from the menu above: ")


For Loops

And here is the new typing exercise for this chapter:

onetoten = range(1, 11)
for count in onetoten:
    print count

and the ever-present output:

1
2
3
4
5
6
7
8
9
10

The output looks awfully familiar but the program code looks different. The first line uses the range function. The range function uses two arguments like this range(start, finish). start is the first number that is produced. finish is one larger than the last number. Note that this program could have been done in a shorter way:

for count in range(1, 11):
    print count

Here are some examples to show what happens with the range command:

>>> range(1, 10)
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> range(-32, -20)
[-32, -31, -30, -29, -28, -27, -26, -25, -24, -23, -22, -21]
>>> range(5,21)
[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
>>> range(5)
[0, 1, 2, 3, 4]
>>> range(21, 5)
[]

The next line for count in onetoten: uses the for control structure. A for control structure looks like for variable in list:. list is gone through starting with the first element of the list and going to the last. As for goes through each element in a list it puts each into variable. That allows variable to be used in each successive time the for loop is run through. Here is another example (you don't have to type this) to demonstrate:

demolist = ['life', 42, 'the universe', 6, 'and', 9, 'everything']
for item in demolist:
    print "The Current item is:",
    print item

The output is:

The Current item is: life
The Current item is: 42
The Current item is: the universe
The Current item is: 6
The Current item is: and
The Current item is: 9
The Current item is: everything

Notice how the for loop goes through and sets item to each element in the list. Notice how if you don't want print to go to the next line add a comma at the end of the statement (i.e. if you want to print something else on that line). So, what is for good for? The first use is to go through all the elements of a list and do something with each of them. Here's a quick way to add up all the elements:

list = [2, 4, 6, 8]
sum = 0
for num in list:
    sum = sum + num
 
print "The sum is:", sum

with the output simply being:

The sum is: 20

Or you could write a program to find out if there are any duplicates in a list like this program does:

list = [4, 5, 7, 8, 9, 1, 0, 7, 10]
list.sort()
prev = list[0]
del list[0]
for item in list:
    if prev == item:
        print "Duplicate of", prev, "found"
    prev = item

and for good measure:

Duplicate of 7 Found

Okay, so how does it work? Here is a special debugging version to help you understand (you don't need to type this in):

l = [4, 5, 7, 8, 9, 1, 0, 7, 10]
print "l = [4, 5, 7, 8, 9, 1, 0, 7, 10]", "\t\tl:", l
l.sort()
print "l.sort()", "\t\tl:", l
prev = l[0]
print "prev = l[0]", "\t\tprev:", prev
del l[0]
print "del l[0]", "\t\tl:", l
for item in l:
    if prev == item:
        print "Duplicate of", prev, "found"
    print "if prev == item:", "\t\tprev:", prev, "\titem:", item
    prev = item
    print "prev = item", "\t\tprev:", prev, "\titem:", item

with the output being:

l = [4, 5, 7, 8, 9, 1, 0, 7, 10]        l: [4, 5, 7, 8, 9, 1, 0, 7, 10]
l.sort()                l: [0, 1, 4, 5, 7, 7, 8, 9, 10]
prev = l[0]             prev: 0
del l[0]                l: [1, 4, 5, 7, 7, 8, 9, 10]
if prev == item:        prev: 0         item: 1
prev = item             prev: 1         item: 1
if prev == item:        prev: 1         item: 4
prev = item             prev: 4         item: 4
if prev == item:        prev: 4         item: 5
prev = item             prev: 5         item: 5
if prev == item:        prev: 5         item: 7
prev = item             prev: 7         item: 7
Duplicate of 7 found
if prev == item:        prev: 7         item: 7
prev = item             prev: 7         item: 7
if prev == item:        prev: 7         item: 8
prev = item             prev: 8         item: 8
if prev == item:        prev: 8         item: 9
prev = item             prev: 9         item: 9
if prev == item:        prev: 9         item: 10
prev = item             prev: 10        item: 10

The reason I put so many print statements in the code was so that you can see what is happening in each line. (By the way, if you can't figure out why a program is not working, try putting in lots of print statements so you can see what is happening.) First the program starts with a boring old list. Next the program sorts the list. This is so that any duplicates get put next to each other. The program then initializes a prev(ious) variable. Next the first element of the list is deleted so that the first item is not incorrectly thought to be a duplicate. Next a for loop is gone into. Each item of the list is checked to see if it is the same as the previous. If it is a duplicate was found. The value of prev is then changed so that the next time the for loop is run through prev is the previous item to the current. Sure enough, the 7 is found to be a duplicate. (Notice how \t is used to print a tab.)

The other way to use for loops is to do something a certain number of times. Here is some code to print out the first 9 numbers of the Fibonacci series:

a = 1
b = 1
for c in range(1, 10):
    print a,
    n = a + b
    a = b
    b = n

with the surprising output:

1 1 2 3 5 8 13 21 34

Everything that can be done with for loops can also be done with while loops but for loops give an easy way to go through all the elements in a list or to do something a certain number of times.

Boolean Expressions

Here is a little example of boolean expressions (you don't have to type it in):

a = 6
b = 7
c = 42
print 1, a == 6
print 2, a == 7
print 3, a == 6 and b == 7
print 4, a == 7 and b == 7
print 5, not a == 7 and b == 7
print 6, a == 7 or b == 7
print 7, a == 7 or b == 6
print 8, not (a == 7 and b == 6)
print 9, not a == 7 and b == 6

With the output being:

1 True
2 False
3 True
4 False
5 True
6 True
7 False
8 True
9 False

What is going on? The program consists of a bunch of funny looking print statements. Each print statement prints a number and an expression. The number is to help keep track of which statement I am dealing with. Notice how each expression ends up being either False or True. In Python, false can also be written as 0 and true as 1.

The lines:

print 1, a == 6
print 2, a == 7

print out a True and a False respectively just as expected since the first is true and the second is false. The third print, print 3, a == 6 and b == 7, is a little different. The operator and means if both the statement before and the statement after are true then the whole expression is true otherwise the whole expression is false. The next line, print 4, a == 7 and b == 7, shows how if part of an and expression is false, the whole thing is false. The behavior of and can be summarized as follows:

expression result
true and true true
true and false false
false and true false
false and false false

Notice that if the first expression is false Python does not check the second expression since it knows the whole expression is false.

The next line, print 5, not a == 7 and b == 7, uses the not operator. not just gives the opposite of the expression. (The expression could be rewritten as print 5, a != 7 and b == 7). Here is the table:

expression result
not true false
not false true

The two following lines, print 6, a == 7 or b == 7 and print 7, a == 7 or b == 6, use the or operator. The or operator returns true if the first expression is true, or if the second expression is true or both are true. If neither are true it returns false. Here's the table:

expression result
true or true true
true or false true
false or true true
false or false false

Notice that if the first expression is true Python doesn't check the second expression since it knows the whole expression is true. This works since or is true if at least one half of the expression is true. The first part is true so the second part could be either false or true, but the whole expression is still true.

The next two lines, print 8, not (a == 7 and b == 6) and print 9, not a == 7 and b == 6, show that parentheses can be used to group expressions and force one part to be evaluated first. Notice that the parentheses changed the expression from false to true. This occurred since the parentheses forced the not to apply to the whole expression instead of just the a == 7 portion.

Here is an example of using a boolean expression:

list = ["Life", "The Universe", "Everything", "Jack", "Jill", "Life", "Jill"]
 
# make a copy of the list. See the More on Lists chapter to explain what [:] means.
copy = list[:]
# sort the copy
copy.sort()
prev = copy[0]
del copy[0]
 
count = 0
 
# go through the list searching for a match
while count < len(copy) and copy[count] != prev:
    prev = copy[count]
    count = count + 1
 
# If a match was not found then count can't be < len
# since the while loop continues while count is < len
# and no match is found
 
if count < len(copy):
    print "First Match:", prev

And here is the output:

First Match: Jill

This program works by continuing to check for match while count < len(copy) and copy[count] is not equal to prev. When either count is greater than the last index of copy or a match has been found the and is no longer true so the loop exits. The if simply checks to make sure that the while exited because a match was found.

The other "trick" of and is used in this example. If you look at the table for and notice that the third entry is "false and won't check". If count >= len(copy) (in other words count < len(copy) is false) then copy[count] is never looked at. This is because Python knows that if the first is false then they can't both be true. This is known as a short circuit and is useful if the second half of the and will cause an error if something is wrong. I used the first expression (count < len(copy)) to check and see if count was a valid index for copy. (If you don't believe me remove the matches "Jill" and "Life", check that it still works and then reverse the order of count < len(copy) and copy[count] != prev to copy[count] != prev and count < len(copy).)

Boolean expressions can be used when you need to check two or more different things at once.

A note on Boolean Operators

A common mistake for people new to programming is a misunderstanding of the way that boolean operators works, which stems from the way the python interpreter reads these expressions. For example, after initially learning about "and " and "or" statements, one might assume that the expression x == ('a' or 'b') would check to see if the variable x was equivalent to one of the strings 'a' or 'b'. This is not so. To see what I'm talking about, start an interactive session with the interpreter and enter the following expressions:

>>> 'a' == ('a' or 'b')
>>> 'b' == ('a' or 'b')
>>> 'a' == ('a' and 'b')
>>> 'b' == ('a' and 'b')

And this will be the unintuitive result:

>>> 'a' == ('a' or 'b')
True
>>> 'b' == ('a' or 'b')
False
>>> 'a' == ('a' and 'b')
False 
>>> 'b' == ('a' and 'b')
True

At this point, the and and or operators seem to be broken. It doesn't make sense that, for the first two expressions, 'a' is equivalent to 'a' or 'b' while 'b' is not. Furthermore, it doesn't make any sense that 'b' is equivalent to 'a' and 'b'. After examining what the interpreter does with boolean operators, these results do in fact exactly what you are asking of them, it's just not the same as what you think you are asking.

When the Python interpreter looks at an or expression, it takes the first statement and checks to see if it is true. If the first statement is true, then Python returns that object's value without checking the second statement. This is because for an or expression, the whole thing is true if one of the values is true; the program does not need to bother with the second statement. On the other hand, if the first value is evaluated as false Python checks the second half and returns that value. That second half determines the truth value of the whole expression since the first half was false. This "laziness" on the part of the interpreter is called "short circuiting" and is a common way of evaluating boolean expressions in many programming languages.

Similarly, for an and expression, Python uses a short circuit technique to speed truth value evaluation. If the first statement is false then the whole thing must be false, so it returns that value. Otherwise if the first value is true it checks the second and returns that value.

One thing to note at this point is that the boolean expression returns a value indicating True or False, but that Python considers a number of different things to have a truth value assigned to them. To check the truth value of any given object x, you can use the function bool(x) to see its truth value. Below is a table with examples of the truth values of various objects:

True False
True False
1 0
Numbers other than zero The string 'None'
Nonempty strings Empty strings
Nonempty lists Empty lists
Nonempty dictionaries Empty dictionaries

Now it is possible to understand the perplexing results we were getting when we tested those boolean expressions before. Let's take a look at what the interpreter "sees" as it goes through that code:

First case:

>>> 'a' == ('a' or 'b')  # Look at parentheses first, so evaluate expression "('a' or 'b')"
                           # 'a' is a nonempty string, so the first value is True
                           # Return that first value: 'a'
>>> 'a' == 'a'           # the string 'a' is equivalent to the string 'a', so expression is True
True

Second case:

>>> 'b' == ('a' or 'b')  # Look at parentheses first, so evaluate expression "('a' or 'b')"
                           # 'a' is a nonempty string, so the first value is True
                           # Return that first value: 'a'
>>> 'b' == 'a'           # the string 'b' is not equivalent to the string 'a', so expression is False
False 

Third case:

>>> 'a' == ('a' and 'b') # Look at parentheses first, so evaluate expression "('a' and 'b')"
                           # 'a' is a nonempty string, so the first value is True, examine second value
                           # 'b' is a nonempty string, so second value is True
                           # Return that second value as result of whole expression: 'b'
>>> 'a' == 'b'           # the string 'a' is not equivalent to the string 'b', so expression is False
False

Fourth case:

>>> 'b' == ('a' and 'b') # Look at parentheses first, so evaluate expression "('a' and 'b')"
                           # 'a' is a nonempty string, so the first value is True, examine second value
                           # 'b' is a nonempty string, so second value is True
                           # Return that second value as result of whole expression: 'b'
>>> 'b' == 'b'           # the string 'b' is equivalent to the string 'b', so expression is True
True 

So Python was really doing its job when it gave those apparently bogus results. As mentioned previously, the important thing is to recognize what value your boolean expression will return when it is evaluated, because it isn't always obvious.

Going back to those initial expressions, this is how you would write them out so they behaved in a way that you want:

>>> 'a' == 'a' or 'a' == 'b' 
True
>>> 'b' == 'a' or 'b' == 'b' 
True
>>> 'a' == 'a' and 'a' == 'b' 
False
>>> 'b' == 'a' and 'b' == 'b'
False

When these comparisons are evaluated they return truth values in terms of True or False, not strings, so we get the proper results.

Examples

password1.py

## This programs asks a user for a name and a password.
# It then checks them to make sure the user is allowed in.
 
name = raw_input("What is your name? ")
password = raw_input("What is the password? ")
if name == "Josh" and password == "Friday":
    print "Welcome Josh"
elif name == "Fred" and password == "Rock":
    print "Welcome Fred"
else:
    print "I don't know you."

Sample runs

What is your name? Josh
What is the password? Friday
Welcome Josh
What is your name? Bill
What is the password? Money
I don't know you.

Exercises

Write a program that has a user guess your name, but they only get 3 chances to do so until the program quits.

Solution

Write a program that has a user guess your name, but they only get 3 chances to do so until the program quits.

print "Try to guess my name!"
count = 3
name = "Tony"
guess = raw_input("What is my name? ")
while count > 1 and guess != name:
    print "You are wrong!"
    guess = raw_input("What is my name? ")
    count = count - 1
 
if guess != name:
    print "You are wrong!" # this message isn't printed in the third chance, so we print it now
    print "You ran out of chances."
    quit
else:
    print "Yes! My name is", name + "!"

Dictionaries

This chapter is about dictionaries. If you open a dictionary, you should notice every entry consists of two parts, a word and the word's definition. The word is the key to finding out what a word means, and what the word means is considered the value for that key. In Python, dictionaries have keys and values. Keys are used to find values. Here is an example of a dictionary in use:

def print_menu():
    print '1. Print Dictionary'
    print '2. Add definition'
    print '3. Remove word'
    print '4. Lookup word'
    print '5. Quit'
    print
 
words = {}
menu_choice = 0
print_menu()
 
while menu_choice != 5:
    menu_choice = input("Type in a number (1-5): ")
    if menu_choice == 1:
        print "Definitions:"
        for x in words.keys():
            print x, ": ", words[x]
        print
    elif menu_choice == 2:
        print "Add definition"
        name = raw_input("Word: ")
        means = raw_input("Definition: ")
        words[name] = means
    elif menu_choice == 3:
        print "Remove word"
        name = raw_input("Word: ")
        if name in words:
            del words[name]
            print name, " was removed."
        else:
            print name, " was not found."
    elif menu_choice == 4:
        print "Lookup Word"
        name = raw_input("Word: ")
        if name in words:
            print "The definition of ", name, " is: ", words[name]
        else:
            print name, "No definition for ", name, " was found."
    elif menu_choice != 5:
        print_menu()

And here is my output:

1. Print Dictionary
2. Add definition
3. Remove word
4. Lookup word
5. Quit

Type in a number (1-5): 2
Add definition
Word: Python
Definition: A snake, a programming language, and a British comedy.
Type in a number (1-5): 2
Add definition
Word: Dictionary
Definition: A book where words are defined.
Type in a number (1-5): 1
Definitions:
Python: A snake, a programming language, and a British comedy.
Dictionary: A book where words are defined.

Type in a number (1-5): 4
Lookup Word
Word: Python
The definition of Python is: A snake, a programming language, and a British comedy.
Type in a number (1-5): 3
Remove Word
Word: Dictionary
Dictionary was removed.
Type in a number (1-5): 1
Definitions:
Python: A snake, a programming language, and a British comedy. 
Type in a number (1-5): 5

This program is similar to the name list from the earlier chapter on lists (note that lists use indexes and dictionaries don't). Here's how the program works:

  • First the function print_menu is defined. print_menu just prints a menu that is later used twice in the program.
  • Next comes the funny looking line words = {}. All that line does is tell Python that words is a dictionary.
  • The next few lines just make the menu work.
for x in words.keys():
    print x, ": ", words[x]
  • This goes through the dictionary and prints all the information. The function words.keys() returns a list that is then used by the for loop. The list returned by keys() is not in any particular order so if you want it in alphabetic order it must be sorted. Similar to lists the statement words[x] is used to access a specific member of the dictionary. Of course in this case x is a string.
  • Next the line words[name] = means adds a word and definition to the dictionary. If name is already in the dictionary means replaces whatever was there before.
if name in words:
    del words[name]
  • See if name is in words and remove it if it is. The expression name in words returns true if name is a key in words but otherwise returns false. The line del words[name] removes the key name and the value associated with that key.
if name in words:
    print "The definition of ", name, " is: ", words[name]
  • Check to see if words has a certain key and if it does prints out the definition associated with it.
  • Lastly if the menu choice is invalid it reprints the menu for your viewing pleasure.

A recap: Dictionaries have keys and values. Keys can be strings or numbers. Keys point to values. Values can be any type of variable (including lists or even dictionaries (those dictionaries or lists of course can contain dictionaries or lists themselves (scary right? :-) )). Here is an example of using a list in a dictionary:

max_points = [25, 25, 50, 25, 100]
assignments = ['hw ch 1', 'hw ch 2', 'quiz   ', 'hw ch 3', 'test']
students = {'#Max': max_points}
 
def print_menu():
    print "1. Add student"
    print "2. Remove student"
    print "3. Print grades"
    print "4. Record grade"
    print "5. Print Menu"
    print "6. Exit"
 
def print_all_grades():
    print '\t',
    for i in range(len(assignments)):
        print assignments[i], '\t',
    print
    keys = students.keys()
    keys.sort()
    for x in keys:
        print x, '\t',
        grades = students[x]
        print_grades(grades)
 
def print_grades(grades):
    for i in range(len(grades)):
        print grades[i], '\t', '\t',
    print
 
print_menu()
menu_choice = 0
while menu_choice != 6:
    print
    menu_choice = input("Menu Choice (1-6): ")
    if menu_choice == 1:
        name = raw_input("Student to add: ")
        students[name] = [0] * len(max_points)
    elif menu_choice == 2:
        name = raw_input("Student to remove: ")
        if name in students:
            del students[name]
        else:
            print "Student:", name, "not found"
    elif menu_choice == 3:
        print_all_grades()
    elif menu_choice == 4:
        print "Record Grade"
        name = raw_input("Student: ")
        if name in students:
            grades = students[name]
            print "Type in the number of the grade to record"
            print "Type a 0 (zero) to exit"
            for i in range(len(assignments)):
                print i + 1, assignments[i], '\t',
            print
            print_grades(grades)
            which = 1234
            while which != -1:
                which = input("Change which Grade: ")
                which = which - 1
                if 0 <= which < len(grades):
                    grade = input("Grade: ")
                    grades[which] = grade
                elif which != -1:
                    print "Invalid Grade Number"
        else:
            print "Student not found"
    elif menu_choice != 6:
        print_menu()

and here is a sample output:

1. Add student
2. Remove student
3. Print grades
4. Record grade
5. Print Menu
6. Exit

Menu Choice (1-6): 3
       hw ch 1         hw ch 2         quiz            hw ch 3         test 
#Max    25              25              50              25              100 

Menu Choice (1-6): 5
1. Add student
2. Remove student
3. Print grades
4. Record grade
5. Print Menu
6. Exit

Menu Choice (1-6): 1
Student to add: Bill

Menu Choice (1-6): 4
Record Grade
Student: Bill
Type in the number of the grade to record
Type a 0 (zero) to exit
1   hw ch 1     2   hw ch 2     3   quiz        4   hw ch 3     5   test 
0               0               0               0               0 
Change which Grade: 1
Grade: 25
Change which Grade: 2
Grade: 24
Change which Grade: 3
Grade: 45
Change which Grade: 4
Grade: 23
Change which Grade: 5
Grade: 95
Change which Grade: 0

Menu Choice (1-6): 3
       hw ch 1         hw ch 2         quiz            hw ch 3         test 
#Max    25              25              50              25              100
Bill    25              24              45              23              95 

Menu Choice (1-6): 6

Heres how the program works. Basically the variable students is a dictionary with the keys being the name of the students and the values being their grades. The first two lines just create two lists. The next line students = {'#Max': max_points} creates a new dictionary with the key {#Max} and the value is set to be [25, 25, 50, 25, 100], since thats what max_points was when the assignment is made (I use the key #Max since # is sorted ahead of any alphabetic characters). Next print_menu is defined. Next the print_all_grades function is defined in the lines:

def print_all_grades():
    print '\t',
    for i in range(len(assignments)):
        print assignments[i], '\t',
    print
    keys = students.keys()
    keys.sort()
    for x in keys:
        print x, '\t',
        grades = students[x]
        print_grades(grades)

Notice how first the keys are gotten out of the students dictionary with the keys function in the line keys = students.keys(). keys is a list so all the functions for lists can be used on it. Next the keys are sorted in the line keys.sort() since it is a list. for is used to go through all the keys. The grades are stored as a list inside the dictionary so the assignment grades = students[x] gives grades the list that is stored at the key x. The function print_grades just prints a list and is defined a few lines later.

The later lines of the program implement the various options of the menu. The line students[name] = [0] * len(max_points) adds a student to the key of their name. The notation [0] * len(max_points) just creates a list of 0's that is the same length as the max_points list.

The remove student entry just deletes a student similar to the telephone book example. The record grades choice is a little more complex. The grades are retrieved in the line grades = students[name] gets a reference to the grades of the student name. A grade is then recorded in the line grades[which] = grade. You may notice that grades is never put back into the students dictionary (as in no students[name] = grades). The reason for the missing statement is that grades is actually another name for students[name] and so changing grades changes student[name].

Dictionaries provide a easy way to link keys to values. This can be used to easily keep track of data that is attached to various keys.

Using Modules

Here's this chapter's typing exercise (name it cal.py). import actually looks for a file named calendar.py and reads it in. If the file is named calendar.py and it sees an "import calendar" it tries to read in itself which works poorly at best.)):

import calendar
year = input("Type in the year number: ")
calendar.prcal(year)

And here is part of the output I got:

Type in the year number: 2001

                                 2001                                  

       January                  February                    March      

Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su
1  2  3  4  5  6  7                1  2  3  4                1  2  3  4     
8  9 10 11 12 13 14       5  6  7  8  9 10 11       5  6  7  8  9 10 11
15 16 17 18 19 20 21      12 13 14 15 16 17 18      12 13 14 15 16 17 18     
22 23 24 25 26 27 28      19 20 21 22 23 24 25      19 20 21 22 23 24 25     
29 30 31                  26 27 28                  26 27 28 29 30 31        

(I skipped some of the output, but I think you get the idea.) So what does the program do? The first line import calendar uses a new command import. The command import loads a module (in this case the calendar module). To see the commands available in the standard modules either look in the library reference for python (if you downloaded it) or go to http://docs.python.org/library/. If you look at the documentation for the calendar module, it lists a function called prcal that prints a calendar for a year. The line calendar.prcal(year) uses this function. In summary to use a module import it and then use module_name.function for functions in the module. Another way to write the program is:

from calendar import prcal
 
year = input("Type in the year number: ")
prcal(year)

This version imports a specific function from a module. Here is another program that uses the Python Library (name it something like clock.py) (press Ctrl and the 'c' key at the same time to terminate the program):

from time import time, ctime
 
prev_time = ""
while True:
    the_time = ctime(time())
    if prev_time != the_time:
        print "The time is:", ctime(time())
        prev_time = the_time

With some output being:

The time is: Sun Aug 20 13:40:04 2000
The time is: Sun Aug 20 13:40:05 2000
The time is: Sun Aug 20 13:40:06 2000
The time is: Sun Aug 20 13:40:07 2000

Traceback (innermost last):
 File "clock.py", line 5, in ?
    the_time = ctime(time())

KeyboardInterrupt

The output is infinite of course so I canceled it (or the output at least continues until Ctrl+C is pressed). The program just does a infinite loop (True is always true, so while True: goes forever) and each time checks to see if the time has changed and prints it if it has. Notice how multiple names after the import statement are used in the line from time import time, ctime.

The Python Library contains many useful functions. These functions give your programs more abilities and many of them can simplify programming in Python.

Exercises

Rewrite the High_low.py program from section Decisions to use a random integer between 0 and 99 instead of the hard-coded 78. Use the Python documentation to find an appropriate module and function to do this.

Solution

Rewrite the High_low.py program from section Decisions to use an random integer between 0 and 99 instead of the hard-coded 78. Use the Python documentation to find an appropriate module and function to do this.

from random import randint
number = randint(0, 99)
guess = -1
while guess != number: 
    guess = input ("Guess a number: ")
    if guess > number:
        print "Too high"
    elif guess < number:
            print "Too low"
print "Just right"


More on Lists

We have already seen lists and how they can be used. Now that you have some more background I will go into more detail about lists. First we will look at more ways to get at the elements in a list and then we will talk about copying them.

Here are some examples of using indexing to access a single element of a list:

>>> some_numbers = ['zero', 'one', 'two', 'three', 'four', 'five']
>>> some_numbers[0]
'zero'
>>> some_numbers[4]
'four'
>>> some_numbers[5]
'five'

All those examples should look familiar to you. If you want the first item in the list just look at index 0. The second item is index 1 and so on through the list. However what if you want the last item in the list? One way could be to use the len() function like some_numbers[len(some_numbers) - 1]. This way works since the len() function always returns the last index plus one. The second from the last would then be some_numbers[len(some_numbers) - 2]. There is an easier way to do this. In Python the last item is always index -1. The second to the last is index -2 and so on. Here are some more examples:

>>> some_numbers[len(some_numbers) - 1]
'five'
>>> some_numbers[len(some_numbers) - 2]
'four'
>>> some_numbers[-1]
'five'
>>> some_numbers[-2]
'four'
>>> some_numbers[-6]
'zero'

Thus any item in the list can be indexed in two ways: from the front and from the back.

Another useful way to get into parts of lists is using slicing. Here is another example to give you an idea what they can be used for:

>>> things = [0, 'Fred', 2, 'S.P.A.M.', 'Stocking', 42, "Jack", "Jill"]
>>> things[0]
0
>>> things[7]
'Jill'
>>> things[0:8]
[0, 'Fred', 2, 'S.P.A.M.', 'Stocking', 42, 'Jack', 'Jill']
>>> things[2:4]
[2, 'S.P.A.M.']
>>> things[4:7]
['Stocking', 42, 'Jack']
>>> things[1:5]
['Fred', 2, 'S.P.A.M.', 'Stocking']

Slicing is used to return part of a list. The slicing operator is in the form things[first_index:last_index]. Slicing cuts the list before the first_index and before the last_index and returns the parts inbetween. You can use both types of indexing:

>>> things[-4:-2]
['Stocking', 42]
>>> things[-4]
'Stocking'
>>> things[-4:6]
['Stocking', 42]

Another trick with slicing is the unspecified index. If the first index is not specified the beginning of the list is assumed. If the last index is not specified the whole rest of the list is assumed. Here are some examples:

>>> things[:2]
[0, 'Fred']
>>> things[-2:]
['Jack', 'Jill']
>>> things[:3]
[0, 'Fred', 2]
>>> things[:-5]
[0, 'Fred', 2]

Here is a (HTML inspired) program example (copy and paste in the poem definition if you want):

poem = ["<B>", "Jack", "and", "Jill", "</B>", "went", "up", "the",
        "hill", "to", "<B>", "fetch", "a", "pail", "of", "</B>",
        "water.", "Jack", "fell", "<B>", "down", "and", "broke",
        "</B>", "his", "crown", "and", "<B>", "Jill", "came",
        "</B>", "tumbling", "after"]
 
def get_bolds(text):
    true = 1
    false = 0
    ## is_bold tells whether or not the we are currently looking at 
    ## a bold section of text.
    is_bold = false
    ## start_block is the index of the start of either an unbolded 
    ## segment of text or a bolded segment.
    start_block = 0
    for index in range(len(text)):
        ## Handle a starting of bold text
        if text[index] == "<B>":
            if is_bold:
                print "Error: Extra Bold"
            ## print "Not Bold:", text[start_block:index]
            is_bold = true
            start_block = index + 1
        ## Handle end of bold text
        ## Remember that the last number in a slice is the index 
        ## after the last index used.
        if text[index] == "</B>":
            if not is_bold:
                print "Error: Extra Close Bold"
            print "Bold [", start_block, ":", index, "]", text[start_block:index]
            is_bold = false
            start_block = index + 1
 
get_bolds(poem)

with the output being:

Bold [ 1 : 4 ] ['Jack', 'and', 'Jill']
Bold [ 11 : 15 ] ['fetch', 'a', 'pail', 'of']
Bold [ 20 : 23 ] ['down', 'and', 'broke']
Bold [ 28 : 30 ] ['Jill', 'came']

The get_bold() function takes in a list that is broken into words and tokens. The tokens that it looks for are <B> which starts the bold text and </B> which ends bold text. The function get_bold() goes through and searches for the start and end tokens.

The next feature of lists is copying them. If you try something simple like:

>>> a = [1, 2, 3]
>>> b = a
>>> print b
[1, 2, 3]
>>> b[1] = 10
>>> print b
[1, 10, 3]
>>> print a
[1, 10, 3]

This probably looks surprising since a modification to b resulted in a being changed as well. What happened is that the statement b = a makes b a reference to a. This means that b can be thought of as another name for a. Hence any modification to b changes a as well. However some assignments don't create two names for one list:

>>> a = [1, 2, 3]
>>> b = a * 2
>>> print a
[1, 2, 3]
>>> print b
[1, 2, 3, 1, 2, 3]
>>> a[1] = 10
>>> print a
[1, 10, 3]
>>> print b
[1, 2, 3, 1, 2, 3]

In this case b is not a reference to a since the expression a * 2 creates a new list. Then the statement b = a * 2 gives b a reference to a * 2 rather than a reference to a. All assignment operations create a reference. When you pass a list as an argument to a function you create a reference as well. Most of the time you don't have to worry about creating references rather than copies. However when you need to make modifications to one list without changing another name of the list you have to make sure that you have actually created a copy.

There are several ways to make a copy of a list. The simplest that works most of the time is the slice operator since it always makes a new list even if it is a slice of a whole list:

>>> a = [1, 2, 3]
>>> b = a[:]
>>> b[1] = 10
>>> print a
[1, 2, 3]
>>> print b
[1, 10, 3]

Taking the slice [:] creates a new copy of the list. However it only copies the outer list. Any sublist inside is still a references to the sublist in the original list. Therefore, when the list contains lists, the inner lists have to be copied as well. You could do that manually but Python already contains a module to do it. You use the deepcopy function of the copy module:

>>> import copy
>>> a = [[1, 2, 3], [4, 5, 6]]
>>> b = a[:]
>>> c = copy.deepcopy(a)
>>> b[0][1] = 10
>>> c[1][1] = 12
>>> print a
[[1, 10, 3], [4, 5, 6]]
>>> print b
[[1, 10, 3], [4, 5, 6]]
>>> print c
[[1, 2, 3], [4, 12, 6]]

First of all notice that a is a list of lists. Then notice that when b[0][1] = 10 is run both a and b are changed, but c is not. This happens because the inner arrays are still references when the slice operator is used. However with deepcopy c was fully copied.

So, should I worry about references every time I use a function or =? The good news is that you only have to worry about references when using dictionaries and lists. Numbers and strings create references when assigned but every operation on numbers and strings that modifies them creates a new copy so you can never modify them unexpectedly. You do have to think about references when you are modifying a list or a dictionary.

By now you are probably wondering why are references used at all? The basic reason is speed. It is much faster to make a reference to a thousand element list than to copy all the elements. The other reason is that it allows you to have a function to modify the inputed list or dictionary. Just remember about references if you ever have some weird problem with data being changed when it shouldn't be.

Revenge of the Strings

And now presenting a cool trick that can be done with strings:

def shout(string):
    for character in string:
        print "Gimme a " + character
        print "'" + character + "'"
shout("Lose")
 
def middle(string):
    print "The middle character is:", string[len(string) / 2]
 
middle("abcdefg")
middle("The Python Programming Language")
middle("Atlanta")

And the output is:

Gimme a L
'L'
Gimme a o
'o'
Gimme a s
's'
Gimme a e
'e'
The middle character is: d
The middle character is: r
The middle character is: a

What these programs demonstrate is that strings are similar to lists in several ways. The shout() function shows that for loops can be used with strings just as they can be used with lists. The middle procedure shows that that strings can also use the len() function and array indexes and slices. Most list features work on strings as well.

The next feature demonstrates some string specific features:

def to_upper(string):
    ## Converts a string to upper case
    upper_case = ""
    for character in string:
        if 'a' <= character <= 'z':
            location = ord(character) - ord('a')
            new_ascii = location + ord('A')
            character = chr(new_ascii)
        upper_case = upper_case + character
    return upper_case
 
print to_upper("This is Text")

with the output being:

THIS IS TEXT

This works because the computer represents the characters of a string as numbers from 0 to 255. Python has a function called ord() (short for ordinal) that returns a character as a number. There is also a corresponding function called chr() that converts a number into a character. With this in mind the program should start to be clear. The first detail is the line: if 'a' <= character <= 'z': which checks to see if a letter is lower case. If it is then the next lines are used. First it is converted into a location so that a = 0, b = 1, c = 2 and so on with the line: location = ord(character) - ord('a'). Next the new value is found with new_ascii = location + ord('A'). This value is converted back to a character that is now upper case.

Now for some interactive typing exercise:

>>> # Integer to String
>>> 2
2
>>> repr(2)
'2'
>>> -123
-123
>>> repr(-123)
'-123'
>>> `123`
'123'
>>> # String to Integer
>>> "23"
'23'
>>> int("23")
23
>>> "23" * 2
'2323'
>>> int("23") * 2
46
>>> # Float to String
>>> 1.23
1.23
>>> repr(1.23)
'1.23'
>>> # Float to Integer
>>> 1.23
1.23
>>> int(1.23)
1
>>> int(-1.23)
-1
>>> # String to Float
>>> float("1.23")
1.23
>>> "1.23" 
'1.23'
>>> float("123")
123.0
>>> `float("1.23")`
'1.23'

If you haven't guessed already the function repr() can convert a integer to a string and the function int() can convert a string to an integer. The function float() can convert a string to a float. The repr() function returns a printable representation of something. `...` converts almost everything into a string, too. Here are some examples of this:

>>> repr(1)
'1'
>>> repr(234.14)
'234.14'
>>> repr([4, 42, 10])
'[4, 42, 10]'
>>> `[4, 42, 10]`
'[4, 42, 10]'

The int() function tries to convert a string (or a float) into a integer. There is also a similar function called float() that will convert a integer or a string into a float. Another function that Python has is the eval() function. The eval() function takes a string and returns data of the type that python thinks it found. For example:

>>> v = eval('123')
>>> print v, type(v)
123 <type 'int'>
>>> v = eval('645.123')
>>> print v, type(v)
645.123 <type 'float'>
>>> v = eval('[1, 2, 3]')
>>> print v, type(v)
[1, 2, 3] <type 'list'>

If you use the eval() function you should check that it returns the type that you expect.

One useful string function is the split() method. Here's an example:

>>> "This is a bunch of words".split()
['This', 'is', 'a', 'bunch', 'of', 'words']
>>> text = "First batch, second batch, third, fourth"
>>> text.split(",")
['First batch', ' second batch', ' third', ' fourth']

Notice how split() converts a string into a list of strings. The string is split by whitespace by default or by the optional argument (in this case a comma). You can also add another argument that tells split() how many times the separator will be used to split the text. For example:

>>> list = text.split(",")
>>> len(list)
4
>>> list[-1]
' fourth'
>>> list = text.split(",", 2)
>>> len(list)
3
>>> list[-1]
' third, fourth'

Slicing strings (and lists)

Strings can be cut into pieces — in the same way as it was shown for lists in the previous chapter — by using the slicing "operator" [:]. The slicing operator works in the same way as before: text[first_index:last_index] (in very rare cases there can be another colon and a third argument, as in the example shown below).

In order not to get confused by the index numbers, it is easiest to see them as clipping places, possibilities to cut a string into parts. Here is an example, which shows the clipping places (in yellow) and their index numbers (red and blue) for a simple text string:

0 1 2 ... -2 -1
text = " S T R I N G "
[: :]

Note that the red indexes are counted from the beginning of the string and the blue ones from the end of the string backwards. (Note that there is no blue -0, which could seem to be logical at the end of the string. Because -0 == 0, (-0 means "beginning of the string" as well.) Now we are ready to use the indexes for slicing operations:

text[1:4] "TRI"
text[:5] "STRIN"
text[:-1] "STRIN"
text[-4:] "RING"
text[2] "R"
text[:] "STRING"
text[::-1] "GNIRTS"

text[1:4] gives us all of the text string between clipping places 1 and 4, "TRI". If you omit one of the [first_index:last_index] arguments, you get the beginning or end of the string as default: text[:5] gives "STRIN". For both first_index and last_index we can use both the red and the blue numbering schema: text[:-1] gives the same as text[:5], because the index -1 is at the same place as 5 in this case. If we do not use an argument containing a colon, the number is treated in a different way: text[2] gives us one character following the second clipping point, "R". The special slicing operation text[:] means "from the beginning to the end" and produces a copy of the entire string (or list, as shown in the previous chapter).

Last but not least, the slicing operation can have a second colon and a third argument, which is interpreted as the "step size": text[::-1] is text from beginning to the end, with a step size of -1. -1 means "every character, but in the other direction". "STRING" backwards is "GNIRTS" (test a step length of 2, if you have not got the point here).

All these slicing operations work with lists as well. In that sense strings are just a special case of lists, where the list elements are single characters. Just remember the concept of clipping places, and the indexes for slicing things will get a lot less confusing.

Examples

# This program requires an excellent understanding of decimal numbers
def to_string(in_int):
    """Converts an integer to a string"""
    out_str = ""
    prefix = ""
    if in_int < 0:
        prefix = "-"
        in_int = -in_int
    while in_int / 10 != 0:
        out_str = chr(ord('0') + in_int % 10) + out_str
        in_int = in_int / 10
    out_str = chr(ord('0') + in_int % 10) + out_str
    return prefix + out_str
 
def to_int(in_str):
    """Converts a string to an integer"""
    out_num = 0
    if in_str[0] == "-":
        multiplier = -1
        in_str = in_str[1:]
    else:
        multiplier = 1
    for x in range(0, len(in_str)):
        out_num = out_num * 10 + ord(in_str[x]) - ord('0')
    return out_num * multiplier
 
print to_string(2)
print to_string(23445)
print to_string(-23445)
print to_int("14234")
print to_int("12345")
print to_int("-3512")

The output is:

2
23445
-23445
14234
12345
-3512

File IO

Here is a simple example of file IO (input/output):

# Write a file
out_file = open("test.txt", "w")
out_file.write("This Text is going to out file\nLook at it and see!")
out_file.close()
 
# Read a file
in_file = open("test.txt", "r")
text = in_file.read()
in_file.close()
 
print text

The output and the contents of the file test.txt are:

This Text is going to out file
Look at it and see!

Notice that it wrote a file called test.txt in the directory that you ran the program from. The \n in the string tells Python to put a newline where it is.

A overview of file IO is:

  • Get a file object with the open function.
  • Read or write to the file object (depending on how it was opened)
  • Close it

The first step is to get a file object. The way to do this is to use the open function. The format is file_object = open(filename, mode) where file_object is the variable to put the file object, filename is a string with the filename, and mode is "r" to read a file or "w" to write a file (and a few others we will skip here). Next the file objects functions can be called. The two most common functions are read and write. The write function adds a string to the end of the file. The read function reads the next thing in the file and returns it as a string. If no argument is given it will return the whole file (as done in the example).

Now here is a new version of the phone numbers program that we made earlier:

def print_numbers(numbers):
    print "Telephone Numbers:"
    for x in numbers.keys():
        print "Name:", x, "\tNumber:", numbers[x]
    print
 
def add_number(numbers, name, number):
    numbers[name] = number
 
def lookup_number(numbers, name):
    if name in numbers:
        return "The number is " + numbers[name]
    else:
        return name + " was not found"
 
def remove_number(numbers, name):
    if name in numbers:
        del numbers[name]
    else:
        print name," was not found"
 
def load_numbers(numbers, filename):
    in_file = open(filename, "r")
    for in_line in in_file:
        in_line = in_line.rstrip('\n') #Eliminate end of line or enter        
        name, number = in_line.split(",")
        numbers[name] = number
    in_file.close()
 
def save_numbers(numbers, filename):
    out_file = open(filename, "w")
    for x in numbers.keys():
        out_file.write(x + "," + numbers[x] + "\n")
    out_file.close()
 
def print_menu():
    print '1. Print Phone Numbers'
    print '2. Add a Phone Number'
    print '3. Remove a Phone Number'
    print '4. Lookup a Phone Number'
    print '5. Load numbers'
    print '6. Save numbers'
    print '7. Quit'
    print
 
phone_list = {}
menu_choice = 0
print_menu()
while True:
    menu_choice = input("Type in a number (1-7): ")
    if menu_choice == 1:
        print_numbers(phone_list)
    elif menu_choice == 2:
        print "Add Name and Number"
        name = raw_input("Name: ")
        phone = raw_input("Number: ")
        add_number(phone_list, name, phone)
    elif menu_choice == 3:
        print "Remove Name and Number"
        name = raw_input("Name: ")
        remove_number(phone_list, name)
    elif menu_choice == 4:
        print "Lookup Number"
        name = raw_input("Name: ")
        print lookup_number(phone_list, name)
    elif menu_choice == 5:
        filename = raw_input("Filename to load: ")
        load_numbers(phone_list, filename)
    elif menu_choice == 6:
        filename = raw_input("Filename to save: ")
        save_numbers(phone_list, filename)
    elif menu_choice == 7:
        break
    else:
        print_menu()
 
print "Goodbye"

Notice that it now includes saving and loading files. Here is some output of my running it twice:

1. Print Phone Numbers
2. Add a Phone Number
3. Remove a Phone Number
4. Lookup a Phone Number
5. Load numbers
6. Save numbers
7. Quit

Type in a number (1-7): 2
Add Name and Number
Name: Jill
Number: 1234
Type in a number (1-7): 2
Add Name and Number
Name: Fred
Number: 4321
Type in a number (1-7): 1
Telephone Numbers:
Name: Jill     Number: 1234
Name: Fred     Number: 4321

Type in a number (1-7): 6
Filename to save: numbers.txt
Type in a number (1-7): 7
Goodbye
1. Print Phone Numbers
2. Add a Phone Number
3. Remove a Phone Number
4. Lookup a Phone Number
5. Load numbers
6. Save numbers
7. Quit

Type in a number (1-7): 5
Filename to load: numbers.txt
Type in a number (1-7): 1
Telephone Numbers:
Name: Jill     Number: 1234
Name: Fred     Number: 4321

Type in a number (1-7): 7
Goodbye

The new portions of this program are:

def load_numbers(numbers, filename):
    in_file = open(filename, "r")
    while True:
        in_line = in_file.readline()
        if not in_line:
            break
        in_line = in_line[:-1]
        name, number = in_line.split(",")
        numbers[name] = number
    in_file.close()
def save_numbers(numbers, filename):
    out_file = open(filename, "w")
    for x in numbers.keys():
        out_file.write(x + "," + numbers[x] + "\n")
    out_file.close()

First we will look at the save portion of the program. First it creates a file object with the command open(filename, "w"). Next it goes through and creates a line for each of the phone numbers with the command out_file.write(x + "," + numbers[x] + "\n"). This writes out a line that contains the name, a comma, the number and follows it by a newline.

The loading portion is a little more complicated. It starts by getting a file object. Then it uses a while True: loop to keep looping until a break statement is encountered. Next it gets a line with the line in_line = in_file.readline(). The readline function will return a empty string when the end of the file is reached. The if statement checks for this and breaks out of the while loop when that happens. Of course if the readline function did not return the newline at the end of the line there would be no way to tell if an empty string was an empty line or the end of the file so the newline is left in what readline returns. Hence we have to get rid of the newline. The line in_line = in_line[:-1] does this for us by dropping the last character. Next the line name, number = in_line.split(",") splits the line at the comma into a name and a number. This is then added to the numbers dictionary.

Exercises

Now modify the grades program from section Dictionaries so that it uses file IO to keep a record of the students.

Solution

Now modify the grades program from section Dictionaries so that it uses file IO to keep a record of the students.

assignments = ['hw ch 1', 'hw ch 2', 'quiz   ', 'hw ch 3', 'test']
students = { }
 
def load_grades(gradesfile):
    inputfile = open(gradesfile, "r")
    grades = [ ]
    while True:
        student_and_grade = inputfile.readline()
        student_and_grade = student_and_grade[:-1]
        if not student_and_grade:
            break
        else:
            studentname, studentgrades = student_and_grade.split(",")
            studentgrades = studentgrades.split(" ")
            students[studentname] = studentgrades
    inputfile.close()
    print "Grades loaded."
 
def save_grades(gradesfile):
    outputfile = open(gradesfile, "w")
    for i in students.keys():
        outputfile.write(i + ",")
        for x in students[i]:
            outputfile.write(x + ",") #added missing comma
        outputfile.write("\n")
    outputfile.close()
    print "Grades saved."
 
def print_menu():
    print "1. Add student"
    print "2. Remove student"
    print "3. Load grades"
    print "4. Record grade"
    print "5. Print grades"
    print "6. Save grades"
    print "7. Print Menu"
    print "9. Quit"
 
def print_all_grades():
    keys = students.keys()
    if keys:
        keys.sort()
        print '\t',
        for i in range(len(assignments)):
            print assignments[i], '\t',
        print
        for x in keys:
            print x, '\t',
            grades = students[x]
            print_grades(grades)
    else:
        print "There are no grades to print."
 
def print_grades(grades):
    for i in range(len(grades)):
        print grades[i], '\t',
    print
 
print_menu()
menu_choice = 0
while menu_choice != 9:
    print
    menu_choice = input("Menu Choice: ")
    if menu_choice == 1:
        name = raw_input("Student to add: ")
        students[name] = [0] * len(assignments)
    elif menu_choice == 2:
        name = raw_input("Student to remove: ")
        if name in students:
            del students[name]
        else:
            print "Student:", name, "not found"
    elif menu_choice == 3:
        gradesfile = raw_input("Load grades from which file? ")
        load_grades(gradesfile)
    elif menu_choice == 4:
        print "Record Grade"
        name = raw_input("Student: ")
        if name in students:
            grades = students[name]
            print "Type in the number of the grade to record"
            print "Type a 0 (zero) to exit"
            for i in range(len(assignments)):
                print i + 1, assignments[i], '\t',
            print
            print_grades(grades)
            which = 1234
            while which != -1:
                which = input("Change which Grade: ")
                which = which - 1
                if 0 <= which < len(grades):
                    grade = raw_input("Grade: ") # Change from input() to raw_input() to avoid an error when saving
                    grades[which] = grade
                elif which != -1:
                    print "Invalid Grade Number"
        else:
            print "Student not found"
    elif menu_choice == 5:
        print_all_grades()
    elif menu_choice == 6:
        gradesfile = raw_input("Save grades to which file? ")
        save_grades(gradesfile)
    elif menu_choice != 9:
        print_menu()


Dealing with the imperfect

...or how to handle errors

So you now have the perfect program, it runs flawlessly, except for one detail, it will crash on invalid user input. Have no fear, for Python has a special control structure for you. It's called try and it tries to do something. Here is an example of a program with a problem:

print "Type Control C or -1 to exit"
number = 1
while number != -1:
    number = int(raw_input("Enter a number: "))
    print "You entered:", number

Notice how when you enter @#& it outputs something like:

Traceback (innermost last):
 File "try_less.py", line 4, in ?
    number = int(raw_input("Enter a number: "))</source>

ValueError: invalid literal for int(): @#&

As you can see the int() function is unhappy with the number @#& (as well it should be). The last line shows what the problem is; Python found a ValueError. How can our program deal with this? What we do is first: put the place where the errors occurs in a try block, and second: tell Python how we want ValueErrors handled. The following program does this:

print "Type Control C or -1 to exit"
number = 1
while number != -1:
    try:
        number = int(raw_input("Enter a number: "))
        print "You entered:", number
    except ValueError:
        print "That was not a number."

Now when we run the new program and give it @#& it tells us "That was not a number." and continues with what it was doing before.

When your program keeps having some error that you know how to handle, put code in a try block, and put the way to handle the error in the except block.

Here is a more complex example of Error Handling.

# Program by Mitchell Aikens 2012
# No copyright.
import math
 
def main():
	success = 0
	while (success == 0):
		try:
			epact()
			success = 1
		except ValueError:
			print "Error. Please enter an integer value."
			year = 0
		except NameError:
			print "Error. Please enter an integer value."
			year = 0
		except SyntaxError:
			print "Error. Please enter an integer value."
			year = 0
		finally:
			print "Program Complete"
 
def epact():
 
    year = int(input("What year is it?\n"))
    C = year/100
    epactval = (8 + (C/4) - C + ((8*C + 13)/25) + 11 * (year%19))%30
    print "The Epact is: ",epactval
 
main()

The program above uses concepts from previous lessons as well as the current lesson. Let's look at the above program in sections.

After we define the function called "main", we tell it that we want to "try" function named "epact". It does so "while" there is no "success". The interpreter then goes to the the line year = int(input("What year is it?\n")). The interpreter takes the value entered by the user and stores it in the variable named "year".

If the value entered is not an integer or a floating point number (which would be converted to an integer by the interpreter), an exception would be raised, and execution of the try block ends, just before success is assigned the value 1.

Let's look at some possible exceptions. the program above does not have an except clause for every possible exception, as there are numerous types or exceptions.

If the value entered for year is an alphabetical character, a NameError exception is raised. In the program above, this is caught by the except NameError: line, and the interpreter executes the print statement below the except NameError:, then it sets the value of "year" to 0 as a precaution, clearing it of any non-numeric number. The interpreter then jumps back to the first line of the while loop, and the process restarts.

The process above would be the same for the other exceptions we have. If an exception is raised, and there is an except clause for it in our program, the interpreter will jump to the statements under the appropriate except clause, and execute them.

The finally statement, is sometimes used in exception handling as well. Think of it as the trump card. Statements underneath the finally clause will be executed regardless of if we raise and exception or not. The finally statement will be executed after any try or except clauses prior to it.

Below is a simpler example where we are not looped, and the finally clause is executed regardless of exceptions.

#Program By Mitchell Aikens 2012
#Not copyright.
 
def main():
    try:
        number = int(input("Please enter a number.\n"))
        half = number/2
        print "Half of the number you entered is ",half
    except NameError:
        print "Error."
    except ValueError:
        print "Error."
    except SyntaxError:
        print "Error."
    finally:
        print "I am executing the finally clause."
 
main()

If we were to enter an alphabetic value for number = int(input("Please enter a number.\n")), the output would be as follows:

Please enter a number.
t
Error.
I am executing the finally clause.

Exercises

Update at least the phone numbers program (in section File IO) so it doesn't crash if a user doesn't enter any data at the menu.

The End

For the moment I recommend looking at The Python 2 Tutorial by Guido van Rossum for more topics. If you have been following this tutorial, you should be able to understand a fair amount of it. If you want to get deeper into Python, Learn Python the Hard Way is a nice on-line textbook, although targeted at people with a more solid programming background. The Python Programming wikibook can be worth looking at, too.

This tutorial is very much a work in progress. Thanks to everyone who has sent me emails about it. I enjoyed reading them, even when I have not always been the best replier.

Happy programming, may it change your life and the world.

Clipboard

To do:

TODO = ['errors',
        'how to make modules',
        'more on loops',
        'more on strings',
        'more on dictionaries',
        'file io',
        'how to use online help',
        'try',
        'pickle',
        'break and continue']

 

FAQ

Question: Can't use programs with input. 
Answer: If you are using IDLE then try using command line. This problem seems to be fixed in IDLE 0.6 and newer. If you are using an older version of IDLE try upgrading to Python 2.0 or newer.
Is there a printable version? 
Yes, see the next question.
Is there a PDF or zipped version? 
Yes, go to http://www.honors.montana.edu/~jjc/easytut for several different versions. Note that this will not always be up to date with the Wikibooks version. The Wikibook can be printed from the print version.
What is the tutorial written with? 
Originally, LaTeX, see the easytut.tex file.
I can't type in programs of more than one line. 
If the programs that you type in run as soon as you are typing them in, you need to edit a file instead of typing them in interactive mode. (Hint: interactive mode is the mode with the >>> prompt in front of it.)
My question is not answered here. 
Ask on the talk page. Please post source code if at all relevant (even, (or maybe especially) if it doesn't work). Helpful things to include are what you were trying to do, what happened, what you expected to happen, error messages, version of Python, Operating System, and whether or not your cat was stepping on the keyboard. (The cat in my house has a fondness for space bars and control keys.)
I want to read it in a different language. 
There are several translations that I know of. One is in Korean and is available at http://home.hanmir.com/~johnsonj/easytut/easytut.html. Another is in Spanish and at http://www.honors.montana.edu/~jjc/easytut/easytut_es/. Another is in Italian and is available at http://www.python.it/doc/tut_begin/index.html. Another is in Greek and available at http://www.honors.montana.edu/~jjc/easytut/easytut_gr/. Several people have said they are doing a translation in other languages such as French, but I never heard back from them. If you have done a translation or know of any translations, please either send it to me or send me a link.
How do I make a GUI in Python? 
You can use either TKinter at http://www.python.org/topics/tkinter/ or WXPython at http://www.wxpython.org/
How do I make a game in Python? 
The best method is probably to use PYgame at http://pygame.org/
How do I make an executable from a Python program? 
Short answer: Python is an interpreted language so that is impossible. Long answer is that something similar to an executable can be created by taking the Python interpreter and the file and joining them together and distributing that. For more on that problem see http://www.python.org/cgi-bin/faqw.py?req=all#4.28. A project that does make executable python files is py2exe - see http://www.py2exe.org.
I need help with the exercises 
Hint, the password program requires two variables, one to keep track of the number of times the password was typed in, and another to keep track of the last password typed in. Also you can download solutions from http://www.honors.montana.edu/~jjc/easytut/
What and when was the last thing changed?
  • 2000-Dec-16, added error handling chapter.
  • 2000-Dec-22, Removed old install procedure.
  • 2001-Jan-16, Fixed bug in program, Added example and data to lists section.
  • 2001-Apr-5, Spelling, grammar, added another how to break programs, url fix for PDF version.
  • 2001-May-13, Added chapter on debugging.
  • 2001-Nov-11, Added exercises, fixed grammar, spelling, and hopefully improved explanations of some things.
  • 2001-Nov-19, Added password exercise, revised references section.
  • 2002-Feb-23, Moved 3 times password exercise, changed l to list in list examples question. Added a new example to Decisions chapter, added two new exercises.
  • 2002-Mar-14, Changed abs to my_abs since python now defines a abs function.
  • 2002-May-15, Added a faq about creating an executable. Added a comment from about the list example. Fixed typos from Axel Kleiboemer.
  • 2002-Jun-14, Changed a program to use while true instead of while 1 to be more clear.
  • 2002-Jul-5, Rewrote functions chapter. Modified fib program to hopefully be clearer.
  • 2003-Jan-3, Added average examples to the decisions chapter.
  • 2003-Jan-19, Added comment about value of a_var. Fixed mistake in average2.py program.
  • 2003-Sep-5, Changed idle instruction to Run->Run Module.
  • 2004-Jun-1, Put on Wikibooks
  • Since then all changes are visible through the Wikibooks version keeping system.


License

The Non-Programmer's Tutorial for Python is licensed under the GNU Free Documentation License. All programming examples in the text are granted to the public domain.

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