\documentstyle[twoside,11pt,myformat]{report}

\title{Python Tutorial}

\input{boilerplate}

\begin{document}

\pagenumbering{roman}

\maketitle

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\begin{abstract}

\noindent

Python is a simple, yet powerful programming language that bridges the

gap between C and shell programming, and is thus ideally suited for

``throw-away programming''

and rapid prototyping. Its syntax is put

together from constructs borrowed from a variety of other languages;

most prominent are influences from ABC, C, Modula-3 and Icon.

The Python interpreter is easily extended with new functions and data

types implemented in C. Python is also suitable as an extension

language for highly customizable C applications such as editors or

window managers.

Python is available for various operating systems, amongst which

several flavors of {\UNIX}, Amoeba, the Apple Macintosh O.S.,

and MS-DOS.

This tutorial introduces the reader informally to the basic concepts

and features of the Python language and system. It helps to have a

Python interpreter handy for hands-on experience, but as the examples

are self-contained, the tutorial can be read off-line as well.

For a description of standard objects and modules, see the {\em Python

Library Reference} document. The {\em Python Reference Manual} gives

a more formal definition of the language.

\end{abstract}

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\tableofcontents

}

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\pagenumbering{arabic}

\chapter{Whetting Your Appetite}

If you ever wrote a large shell script, you probably know this

feeling: you'd love to add yet another feature, but it's already so

slow, and so big, and so complicated; or the feature involves a system

call or other function that is only accessible from C \ldots Usually

the problem at hand isn't serious enough to warrant rewriting the

script in C; perhaps because the problem requires variable-length

strings or other data types (like sorted lists of file names) that are

easy in the shell but lots of work to implement in C; or perhaps just

because you're not sufficiently familiar with C.

In such cases, Python may be just the language for you. Python is

simple to use, but it is a real programming language, offering much

more structure and support for large programs than the shell has. On

the other hand, it also offers much more error checking than C, and,

being a {\em very-high-level language}, it has high-level data types

built in, such as flexible arrays and dictionaries that would cost you

days to implement efficiently in C. Because of its more general data

types Python is applicable to a much larger problem domain than {\em

Awk} or even {\em Perl}, yet many things are at least as easy in

Python as in those languages.

Python allows you to split up your program in modules that can be

reused in other Python programs. It comes with a large collection of

standard modules that you can use as the basis of your programs --- or

as examples to start learning to program in Python. There are also

built-in modules that provide things like file I/O, system calls,

sockets, and even a generic interface to window systems (STDWIN).

Python is an interpreted language, which can save you considerable time

during program development because no compilation and linking is

necessary. The interpreter can be used interactively, which makes it

easy to experiment with features of the language, to write throw-away

programs, or to test functions during bottom-up program development.

It is also a handy desk calculator.

Python allows writing very compact and readable programs. Programs

written in Python are typically much shorter than equivalent C

programs, for several reasons:

\begin{itemize}

\item

the high-level data types allow you to express complex operations in a

single statement;

\item

statement grouping is done by indentation instead of begin/end

brackets;

\item

no variable or argument declarations are necessary.

\end{itemize}

Python is {\em extensible}: if you know how to program in C it is easy

to add a new built-in

function or

module to the interpreter, either to

perform critical operations at maximum speed, or to link Python

programs to libraries that may only be available in binary form (such

as a vendor-specific graphics library). Once you are really hooked,

you can link the Python interpreter into an application written in C

and use it as an extension or command language for that application.

By the way, the language is named after the BBC show ``Monty

Python's Flying Circus'' and has nothing to do with nasty reptiles...

\section{Where From Here}

Now that you are all excited about Python, you'll want to examine it

in some more detail. Since the best way to learn a language is

using it, you are invited here to do so.

In the next chapter, the mechanics of using the interpreter are

explained. This is rather mundane information, but essential for

trying out the examples shown later.

The rest of the tutorial introduces various features of the Python

language and system though examples, beginning with simple

expressions, statements and data types, through functions and modules,

and finally touching upon advanced concepts like exceptions

and user-defined classes.

When you're through with the tutorial (or just getting bored), you

should read the Library Reference, which gives complete (though terse)

reference material about built-in and standard types, functions and

modules that can save you a lot of time when writing Python programs.

\chapter{Using the Python Interpreter}

\section{Invoking the Interpreter}

The Python interpreter is usually installed as {\tt /usr/local/bin/python}

on those machines where it is available; putting {\tt /usr/local/bin} in

your {\UNIX} shell's search path makes it possible to start it by

typing the command

\bcode\begin{verbatim}

python

\end{verbatim}\ecode

%

to the shell. Since the choice of the directory where the interpreter

lives is an installation option, other places are possible; check with

your local Python guru or system administrator. (E.g., {\tt

/usr/local/python} is a popular alternative location.)

The interpreter operates somewhat like the {\UNIX} shell: when called

with standard input connected to a tty device, it reads and executes

commands interactively; when called with a file name argument or with

a file as standard input, it reads and executes a {\em script} from

that file.

A third way of starting the interpreter is

``{\tt python -c command [arg] ...}'', which

executes the statement(s) in {\tt command}, analogous to the shell's

{\tt -c} option. Since Python statements often contain spaces or other

characters that are special to the shell, it is best to quote {\tt

command} in its entirety with double quotes.

Note that there is a difference between ``{\tt python file}'' and

``{\tt python $<$file}''. In the latter case, input requests from the

program, such as calls to {\tt input()} and {\tt raw_input()}, are

satisfied from {\em file}. Since this file has already been read

until the end by the parser before the program starts executing, the

program will encounter EOF immediately. In the former case (which is

usually what you want) they are satisfied from whatever file or device

is connected to standard input of the Python interpreter.

When a script file is used, it is sometimes useful to be able to run

the script and enter interactive mode afterwards. This can be done by

passing {\tt -i} before the script. (This does not work if the script

is read from standard input, for the same reason as explained in the

previous paragraph.)

\subsection{Argument Passing}

When known to the interpreter, the script name and additional

arguments thereafter are passed to the script in the variable {\tt

sys.argv}, which is a list of strings. Its length is at least one;

when no script and no arguments are given, {\tt sys.argv[0]} is an

empty string. When the script name is given as {\tt '-'} (meaning

standard input), {\tt sys.argv[0]} is set to {\tt '-'}. When {\tt -c

command} is used, {\tt sys.argv[0]} is set to {\tt '-c'}. Options

found after {\tt -c command} are not consumed by the Python

interpreter's option processing but left in {\tt sys.argv} for the

command to handle.

\subsection{Interactive Mode}

When commands are read from a tty, the interpreter is said to be in

{\em interactive\ mode}. In this mode it prompts for the next command

with the {\em primary\ prompt}, usually three greater-than signs ({\tt

>>>}); for continuation lines it prompts with the

{\em secondary\ prompt},

by default three dots ({\tt ...}). Typing an EOF (Control-D)

at the primary prompt causes the interpreter to exit with a zero exit

status.

The interpreter prints a welcome message stating its version number

and a copyright notice before printing the first prompt, e.g.:

\bcode\begin{verbatim}

python

Python 1.2 (Mar 14 1995)

Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam

>>>

\end{verbatim}\ecode

\section{The Interpreter and its Environment}

\subsection{Error Handling}

When an error occurs, the interpreter prints an error

message and a stack trace. In interactive mode, it then returns to

the primary prompt; when input came from a file, it exits with a

nonzero exit status after printing

the stack trace. (Exceptions handled by an {\tt except} clause in a

{\tt try} statement are not errors in this context.) Some errors are

unconditionally fatal and cause an exit with a nonzero exit; this

applies to internal inconsistencies and some cases of running out of

memory. All error messages are written to the standard error stream;

normal output from the executed commands is written to standard

output.

Typing the interrupt character (usually Control-C or DEL) to the

primary or secondary prompt cancels the input and returns to the

primary prompt.%

\footnote{

A problem with the GNU Readline package may prevent this.

}

Typing an interrupt while a command is executing raises the {\tt

KeyboardInterrupt} exception, which may be handled by a {\tt try}

statement.

\subsection{The Module Search Path}

When a module named {\tt spam} is imported, the interpreter searches

for a file named {\tt spam.py} in the list of directories specified by

the environment variable {\tt PYTHONPATH}. It has the same syntax as

the {\UNIX} shell variable {\tt PATH}, i.e., a list of colon-separated

directory names. When {\tt PYTHONPATH} is not set, or when the file

is not found there, the search continues in an installation-dependent

default path, usually {\tt .:/usr/local/lib/python}.

Actually, modules are searched in the list of directories given by the

variable {\tt sys.path} which is initialized from {\tt PYTHONPATH} and

the installation-dependent default. This allows Python programs that

know what they're doing to modify or replace the module search path.

See the section on Standard Modules later.

\subsection{``Compiled'' Python files}

As an important speed-up of the start-up time for short programs that

use a lot of standard modules, if a file called {\tt spam.pyc} exists

in the directory where {\tt spam.py} is found, this is assumed to

contain an already-``compiled'' version of the module {\tt spam}. The

modification time of the version of {\tt spam.py} used to create {\tt

spam.pyc} is recorded in {\tt spam.pyc}, and the file is ignored if

these don't match.

Whenever {\tt spam.py} is successfully compiled, an attempt is made to

write the compiled version to {\tt spam.pyc}. It is not an error if

this attempt fails; if for any reason the file is not written

completely, the resulting {\tt spam.pyc} file will be recognized as

invalid and thus ignored later.

\subsection{Executable Python scripts}

On BSD'ish {\UNIX} systems, Python scripts can be made directly

executable, like shell scripts, by putting the line

\bcode\begin{verbatim}

#! /usr/local/bin/python

\end{verbatim}\ecode

%

(assuming that's the name of the interpreter) at the beginning of the

script and giving the file an executable mode. The {\tt \#!} must be

the first two characters of the file.

\subsection{The Interactive Startup File}

When you use Python interactively, it is frequently handy to have some

standard commands executed every time the interpreter is started. You

can do this by setting an environment variable named {\tt

PYTHONSTARTUP} to the name of a file containing your start-up

commands. This is similar to the {\tt .profile} feature of the UNIX

shells.

This file is only read in interactive sessions, not when Python reads

commands from a script, and not when {\tt /dev/tty} is given as the

explicit source of commands (which otherwise behaves like an

interactive session). It is executed in the same name space where

interactive commands are executed, so that objects that it defines or

imports can be used without qualification in the interactive session.

You can also change the prompts {\tt sys.ps1} and {\tt sys.ps2} in

this file.

If you want to read an additional start-up file from the current

directory, you can program this in the global start-up file, e.g.

\verb\execfile('.pythonrc')\. If you want to use the startup file

in a script, you must write this explicitly in the script, e.g.

\verb\import os;\ \verb\execfile(os.environ['PYTHONSTARTUP'])\.

\section{Interactive Input Editing and History Substitution}

Some versions of the Python interpreter support editing of the current

input line and history substitution, similar to facilities found in

the Korn shell and the GNU Bash shell. This is implemented using the

{\em GNU\ Readline} library, which supports Emacs-style and vi-style

editing. This library has its own documentation which I won't

duplicate here; however, the basics are easily explained.

Perhaps the quickest check to see whether command line editing is

supported is typing Control-P to the first Python prompt you get. If

it beeps, you have command line editing. If nothing appears to

happen, or if \verb/^P/ is echoed, you can skip the rest of this

section.

\subsection{Line Editing}

If supported, input line editing is active whenever the interpreter

prints a primary or secondary prompt. The current line can be edited

using the conventional Emacs control characters. The most important

of these are: C-A (Control-A) moves the cursor to the beginning of the

line, C-E to the end, C-B moves it one position to the left, C-F to

the right. Backspace erases the character to the left of the cursor,

C-D the character to its right. C-K kills (erases) the rest of the

line to the right of the cursor, C-Y yanks back the last killed

string. C-underscore undoes the last change you made; it can be

repeated for cumulative effect.

\subsection{History Substitution}

History substitution works as follows. All non-empty input lines

issued are saved in a history buffer, and when a new prompt is given

you are positioned on a new line at the bottom of this buffer. C-P

moves one line up (back) in the history buffer, C-N moves one down.

Any line in the history buffer can be edited; an asterisk appears in

front of the prompt to mark a line as modified. Pressing the Return

key passes the current line to the interpreter. C-R starts an

incremental reverse search; C-S starts a forward search.

\subsection{Key Bindings}

The key bindings and some other parameters of the Readline library can

be customized by placing commands in an initialization file called

{\tt \$HOME/.inputrc}. Key bindings have the form

\bcode\begin{verbatim}

key-name: function-name

\end{verbatim}\ecode

%

or

\bcode\begin{verbatim}

"string": function-name

\end{verbatim}\ecode

%

and options can be set with

\bcode\begin{verbatim}

set option-name value

\end{verbatim}\ecode

%

For example:

\bcode\begin{verbatim}

# I prefer vi-style editing:

set editing-mode vi

# Edit using a single line:

set horizontal-scroll-mode On

# Rebind some keys:

Meta-h: backward-kill-word

"\C-u": universal-argument

"\C-x\C-r": re-read-init-file

\end{verbatim}\ecode

%

Note that the default binding for TAB in Python is to insert a TAB

instead of Readline's default filename completion function. If you

insist, you can override this by putting

\bcode\begin{verbatim}

TAB: complete

\end{verbatim}\ecode

%

in your {\tt \$HOME/.inputrc}. (Of course, this makes it hard to type

indented continuation lines...)

\subsection{Commentary}

This facility is an enormous step forward compared to previous

versions of the interpreter; however, some wishes are left: It would

be nice if the proper indentation were suggested on continuation lines

(the parser knows if an indent token is required next). The

completion mechanism might use the interpreter's symbol table. A

command to check (or even suggest) matching parentheses, quotes etc.

would also be useful.

\chapter{An Informal Introduction to Python}

In the following examples, input and output are distinguished by the

presence or absence of prompts ({\tt >>>} and {\tt ...}): to repeat

the example, you must type everything after the prompt, when the

prompt appears; lines that do not begin with a prompt are output from

the interpreter.%

\footnote{

I'd prefer to use different fonts to distinguish input

from output, but the amount of LaTeX hacking that would require

is currently beyond my ability.

}

Note that a secondary prompt on a line by itself in an example means

you must type a blank line; this is used to end a multi-line command.

\section{Using Python as a Calculator}

Let's try some simple Python commands. Start the interpreter and wait

for the primary prompt, {\tt >>>}. (It shouldn't take long.)

\subsection{Numbers}

The interpreter acts as a simple calculator: you can type an

expression at it and it will write the value. Expression syntax is

straightforward: the operators {\tt +}, {\tt -}, {\tt *} and {\tt /}

work just like in most other languages (e.g., Pascal or C); parentheses

can be used for grouping. For example:

\bcode\begin{verbatim}

>>> 2+2

4

>>> # This is a comment

... 2+2

4

>>> 2+2 # and a comment on the same line as code

4

>>> (50-5*6)/4

5

>>> # Integer division returns the floor:

... 7/3

2

>>> 7/-3

-3

>>>

\end{verbatim}\ecode

%

Like in C, the equal sign ({\tt =}) is used to assign a value to a

variable. The value of an assignment is not written:

\bcode\begin{verbatim}

>>> width = 20

>>> height = 5*9

>>> width * height

900

>>>

\end{verbatim}\ecode

%

A value can be assigned to several variables simultaneously:

\bcode\begin{verbatim}

>>> x = y = z = 0 # Zero x, y and z

>>> x

0

>>> y

0

>>> z

0

>>>

\end{verbatim}\ecode

%

There is full support for floating point; operators with mixed type

operands convert the integer operand to floating point:

\bcode\begin{verbatim}

>>> 4 * 2.5 / 3.3

3.0303030303

>>> 7.0 / 2

3.5

>>>

\end{verbatim}\ecode

\subsection{Strings}

Besides numbers, Python can also manipulate strings, enclosed in

single quotes or double quotes:

\bcode\begin{verbatim}

>>> 'spam eggs'

'spam eggs'

>>> 'doesn\'t'

"doesn't"

>>> "doesn't"

"doesn't"

>>> '"Yes," he said.'

'"Yes," he said.'

>>> "\"Yes,\" he said."

'"Yes," he said.'

>>> '"Isn\'t," she said.'

'"Isn\'t," she said.'

>>>

\end{verbatim}\ecode

%

Strings are written the same way as they are typed for input: inside

quotes and with quotes and other funny characters escaped by backslashes,

to show the precise value. The string is enclosed in double quotes if

the string contains a single quote and no double quotes, else it's

enclosed in single quotes. (The {\tt print} statement, described later,

can be used to write strings without quotes or escapes.)

Strings can be concatenated (glued together) with the {\tt +}

operator, and repeated with {\tt *}:

\bcode\begin{verbatim}

>>> word = 'Help' + 'A'

>>> word

'HelpA'

>>> '<' + word*5 + '>'

'<HelpAHelpAHelpAHelpAHelpA>'

>>>

\end{verbatim}\ecode

%

Strings can be subscripted (indexed); like in C, the first character of

a string has subscript (index) 0.

There is no separate character type; a character is simply a string of

size one. Like in Icon, substrings can be specified with the {\em

slice} notation: two indices separated by a colon.

\bcode\begin{verbatim}

>>> word[4]

'A'

>>> word[0:2]

'He'

>>> word[2:4]

'lp'

>>>

\end{verbatim}\ecode

%

Slice indices have useful defaults; an omitted first index defaults to

zero, an omitted second index defaults to the size of the string being

sliced.

\bcode\begin{verbatim}

>>> word[:2] # The first two characters

'He'

>>> word[2:] # All but the first two characters

'lpA'

>>>

\end{verbatim}\ecode

%

Here's a useful invariant of slice operations: \verb\s[:i] + s[i:]\

equals \verb\s\.

\bcode\begin{verbatim}

>>> word[:2] + word[2:]

'HelpA'

>>> word[:3] + word[3:]

'HelpA'

>>>

\end{verbatim}\ecode

%

Degenerate slice indices are handled gracefully: an index that is too

large is replaced by the string size, an upper bound smaller than the

lower bound returns an empty string.

\bcode\begin{verbatim}

>>> word[1:100]

'elpA'

>>> word[10:]

''

>>> word[2:1]

''

>>>

\end{verbatim}\ecode

%

Indices may be negative numbers, to start counting from the right.

For example:

\bcode\begin{verbatim}

>>> word[-1] # The last character

'A'

>>> word[-2] # The last-but-one character

'p'

>>> word[-2:] # The last two characters

'pA'

>>> word[:-2] # All but the last two characters

'Hel'

>>>

\end{verbatim}\ecode

%

But note that -0 is really the same as 0, so it does not count from

the right!

\bcode\begin{verbatim}

>>> word[-0] # (since -0 equals 0)

'H'

>>>

\end{verbatim}\ecode

%

Out-of-range negative slice indices are truncated, but don't try this

for single-element (non-slice) indices:

\bcode\begin{verbatim}

>>> word[-100:]

'HelpA'

>>> word[-10] # error

Traceback (innermost last):

File "<stdin>", line 1

IndexError: string index out of range

>>>

\end{verbatim}\ecode

%

The best way to remember how slices work is to think of the indices as

pointing {\em between} characters, with the left edge of the first

character numbered 0. Then the right edge of the last character of a

string of {\tt n} characters has index {\tt n}, for example:

\bcode\begin{verbatim}

+---+---+---+---+---+

| H | e | l | p | A |

+---+---+---+---+---+

0 1 2 3 4 5

-5 -4 -3 -2 -1

\end{verbatim}\ecode

%

The first row of numbers gives the position of the indices 0...5 in

the string; the second row gives the corresponding negative indices.

The slice from \verb\i\ to \verb\j\ consists of all characters between

the edges labeled \verb\i\ and \verb\j\, respectively.

For nonnegative indices, the length of a slice is the difference of

the indices, if both are within bounds, e.g., the length of

\verb\word[1:3]\ is 2.

The built-in function {\tt len()} returns the length of a string:

\bcode\begin{verbatim}

>>> s = 'supercalifragilisticexpialidocious'

>>> len(s)

34

>>>

\end{verbatim}\ecode

\subsection{Lists}

Python knows a number of {\em compound} data types, used to group

together other values. The most versatile is the {\em list}, which

can be written as a list of comma-separated values (items) between

square brackets. List items need not all have the same type.

\bcode\begin{verbatim}

>>> a = ['spam', 'eggs', 100, 1234]

>>> a

['spam', 'eggs', 100, 1234]

>>>

\end{verbatim}\ecode

%

Like string indices, list indices start at 0, and lists can be sliced,

concatenated and so on:

\bcode\begin{verbatim}

>>> a[0]

'spam'

>>> a[3]

1234

>>> a[-2]

100

>>> a[1:-1]

['eggs', 100]

>>> a[:2] + ['bacon', 2*2]

['spam', 'eggs', 'bacon', 4]

>>> 3*a[:3] + ['Boe!']

['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boe!']

>>>

\end{verbatim}\ecode

%

Unlike strings, which are {\em immutable}, it is possible to change

individual elements of a list:

\bcode\begin{verbatim}

>>> a

['spam', 'eggs', 100, 1234]

>>> a[2] = a[2] + 23

>>> a

['spam', 'eggs', 123, 1234]

>>>

\end{verbatim}\ecode

%

Assignment to slices is also possible, and this can even change the size

of the list:

\bcode\begin{verbatim}

>>> # Replace some items:

... a[0:2] = [1, 12]

>>> a

[1, 12, 123, 1234]

>>> # Remove some:

... a[0:2] = []

>>> a

[123, 1234]

>>> # Insert some:

... a[1:1] = ['bletch', 'xyzzy']

>>> a

[123, 'bletch', 'xyzzy', 1234]

>>> a[:0] = a # Insert (a copy of) itself at the beginning

>>> a

[123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy', 1234]

>>>

\end{verbatim}\ecode

%

The built-in function {\tt len()} also applies to lists:

\bcode\begin{verbatim}

>>> len(a)

8

>>>

\end{verbatim}\ecode

%

It is possible to nest lists (create lists containing other lists),

for example:

\bcode\begin{verbatim}

>>> q = [2, 3]

>>> p = [1, q, 4]

>>> len(p)

3

>>> p[1]

[2, 3]

>>> p[1][0]

2

>>> p[1].append('xtra') # See section 5.1

>>> p

[1, [2, 3, 'xtra'], 4]

>>> q

[2, 3, 'xtra']

>>>

\end{verbatim}\ecode

%

Note that in the last example, {\tt p[1]} and {\tt q} really refer to

the same object! We'll come back to {\em object semantics} later.

\section{First Steps Towards Programming}

Of course, we can use Python for more complicated tasks than adding

two and two together. For instance, we can write an initial

subsequence of the {\em Fibonacci} series as follows:

\bcode\begin{verbatim}

>>> # Fibonacci series:

... # the sum of two elements defines the next

... a, b = 0, 1

>>> while b < 10:

... print b

... a, b = b, a+b

...

1

1

2

3

5

8

>>>

\end{verbatim}\ecode

%

This example introduces several new features.

\begin{itemize}

\item

The first line contains a {\em multiple assignment}: the variables

{\tt a} and {\tt b} simultaneously get the new values 0 and 1. On the

last line this is used again, demonstrating that the expressions on

the right-hand side are all evaluated first before any of the

assignments take place.

\item

The {\tt while} loop executes as long as the condition (here: {\tt b <

10}) remains true. In Python, like in C, any non-zero integer value is

true; zero is false. The condition may also be a string or list value,

in fact any sequence; anything with a non-zero length is true, empty

sequences are false. The test used in the example is a simple

comparison. The standard comparison operators are written the same as

in C: {\tt <}, {\tt >}, {\tt ==}, {\tt <=}, {\tt >=} and {\tt !=}.

\item

The {\em body} of the loop is {\em indented}: indentation is Python's

way of grouping statements. Python does not (yet!) provide an

intelligent input line editing facility, so you have to type a tab or

space(s) for each indented line. In practice you will prepare more

complicated input for Python with a text editor; most text editors have

an auto-indent facility. When a compound statement is entered

interactively, it must be followed by a blank line to indicate

completion (since the parser cannot guess when you have typed the last

line).

\item

The {\tt print} statement writes the value of the expression(s) it is

given. It differs from just writing the expression you want to write

(as we did earlier in the calculator examples) in the way it handles

multiple expressions and strings. Strings are printed without quotes,

and a space is inserted between items, so you can format things nicely,

like this:

\bcode\begin{verbatim}

>>> i = 256*256

>>> print 'The value of i is', i

The value of i is 65536

>>>

\end{verbatim}\ecode

%

A trailing comma avoids the newline after the output:

\bcode\begin{verbatim}

>>> a, b = 0, 1

>>> while b < 1000:

... print b,

... a, b = b, a+b

...

1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987

>>>

\end{verbatim}\ecode

%

Note that the interpreter inserts a newline before it prints the next

prompt if the last line was not completed.

\end{itemize}

\chapter{More Control Flow Tools}

Besides the {\tt while} statement just introduced, Python knows the

usual control flow statements known from other languages, with some

twists.

\section{If Statements}

Perhaps the most well-known statement type is the {\tt if} statement.

For example:

\bcode\begin{verbatim}

>>> if x < 0:

... x = 0

... print 'Negative changed to zero'

... elif x == 0:

... print 'Zero'

... elif x == 1:

... print 'Single'

... else:

... print 'More'

...

\end{verbatim}\ecode

%

There can be zero or more {\tt elif} parts, and the {\tt else} part is

optional. The keyword `{\tt elif}' is short for `{\tt else if}', and is

useful to avoid excessive indentation. An {\tt if...elif...elif...}

sequence is a substitute for the {\em switch} or {\em case} statements

found in other languages.

\section{For Statements}

The {\tt for} statement in Python differs a bit from what you may be

used to in C or Pascal. Rather than always iterating over an

arithmetic progression of numbers (like in Pascal), or leaving the user

completely free in the iteration test and step (as C), Python's {\tt

for} statement iterates over the items of any sequence (e.g., a list

or a string), in the order that they appear in the sequence. For

example (no pun intended):

\bcode\begin{verbatim}

>>> # Measure some strings:

... a = ['cat', 'window', 'defenestrate']

>>> for x in a:

... print x, len(x)

...

cat 3

window 6

defenestrate 12

>>>

\end{verbatim}\ecode

%

It is not safe to modify the sequence being iterated over in the loop

(this can only happen for mutable sequence types, i.e., lists). If

you need to modify the list you are iterating over, e.g., duplicate

selected items, you must iterate over a copy. The slice notation

makes this particularly convenient:

\bcode\begin{verbatim}

>>> for x in a[:]: # make a slice copy of the entire list

... if len(x) > 6: a.insert(0, x)

...

>>> a

['defenestrate', 'cat', 'window', 'defenestrate']

>>>

\end{verbatim}\ecode

\section{The {\tt range()} Function}

If you do need to iterate over a sequence of numbers, the built-in

function {\tt range()} comes in handy. It generates lists containing

arithmetic progressions, e.g.:

\bcode\begin{verbatim}

>>> range(10)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>>

\end{verbatim}\ecode

%

The given end point is never part of the generated list; {\tt range(10)}

generates a list of 10 values, exactly the legal indices for items of a

sequence of length 10. It is possible to let the range start at another

number, or to specify a different increment (even negative):

\bcode\begin{verbatim}

>>> range(5, 10)

[5, 6, 7, 8, 9]

>>> range(0, 10, 3)

[0, 3, 6, 9]

>>> range(-10, -100, -30)

[-10, -40, -70]

>>>

\end{verbatim}\ecode

%

To iterate over the indices of a sequence, combine {\tt range()} and

{\tt len()} as follows:

\bcode\begin{verbatim}

>>> a = ['Mary', 'had', 'a', 'little', 'lamb']

>>> for i in range(len(a)):

... print i, a[i]

...

0 Mary

1 had

2 a

3 little

4 lamb

>>>

\end{verbatim}\ecode

\section{Break and Continue Statements, and Else Clauses on Loops}

The {\tt break} statement, like in C, breaks out of the smallest

enclosing {\tt for} or {\tt while} loop.

The {\tt continue} statement, also borrowed from C, continues with the

next iteration of the loop.

Loop statements may have an {\tt else} clause; it is executed when the

loop terminates through exhaustion of the list (with {\tt for}) or when

the condition becomes false (with {\tt while}), but not when the loop is

terminated by a {\tt break} statement. This is exemplified by the

following loop, which searches for prime numbers:

\bcode\begin{verbatim}

>>> for n in range(2, 10):

... for x in range(2, n):

... if n % x == 0:

... print n, 'equals', x, '*', n/x

... break

... else:

... print n, 'is a prime number'

...

2 is a prime number

3 is a prime number

4 equals 2 * 2

5 is a prime number

6 equals 2 * 3

7 is a prime number

8 equals 2 * 4

9 equals 3 * 3

>>>

\end{verbatim}\ecode

\section{Pass Statements}

The {\tt pass} statement does nothing.

It can be used when a statement is required syntactically but the

program requires no action.

For example:

\bcode\begin{verbatim}

>>> while 1:

... pass # Busy-wait for keyboard interrupt

...

\end{verbatim}\ecode

\section{Defining Functions}

We can create a function that writes the Fibonacci series to an