\documentclass{manual}

% Things to do:

% Add a section on file I/O

% Write a chapter entitled ``Some Useful Modules''

% --regex, math+cmath

% Should really move the Python startup file info to an appendix

\title{Python Tutorial}

\input{boilerplate}

\begin{document}

\maketitle

\ifhtml

\chapter*{Front Matter\label{front}}

\fi

\input{copyright}

\begin{abstract}

\noindent

Python is an easy to learn, powerful programming language. It has

efficient high-level data structures and a simple but effective

approach to object-oriented programming. Python's elegant syntax and

dynamic typing, together with its interpreted nature, make it an ideal

language for scripting and rapid application development in many areas

on most platforms.

The Python interpreter and the extensive standard library are freely

available in source or binary form for all major platforms from the

Python web site, \url{http://www.python.org}, and can be freely

distributed. The same site also contains distributions of and

pointers to many free third party Python modules, programs and tools,

and additional documentation.

The Python interpreter is easily extended with new functions and data

types implemented in C or \Cpp{} (or other languages callable from C).

Python is also suitable as an extension language for customizable

applications.

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 all examples are

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

For a description of standard objects and modules, see the

\emph{Python Library Reference} document. The \emph{Python Reference

Manual} gives a more formal definition of the language. To write

extensions in C or \Cpp{}, read the \emph{Extending and Embedding} and

\emph{Python/C API} manuals. There are also several books covering

Python in depth.

This tutorial does not attempt to be comprehensive and cover every

single feature, or even every commonly used feature. Instead, it

introduces many of Python's most noteworthy features, and will give

you a good idea of the language's flavor and style. After reading it,

you will be able to read and write Python modules and programs, and

you will be ready to learn more about the various Python library

modules described in the \emph{Python Library Reference}.

\end{abstract}

\tableofcontents

\chapter{Whetting Your Appetite \label{intro}}

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 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 you're not

sufficiently familiar with C.

Another situation: perhaps you have to work with several C libraries,

and the usual C write/compile/test/re-compile cycle is too slow. You

need to develop software more quickly. Possibly perhaps you've

written a program that could use an extension language, and you don't

want to design a language, write and debug an interpreter for it, then

tie it into your application.

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 \emph{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

\emph{Awk} or even \emph{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 interfaces to GUI toolkits like Tk.

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 \emph{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. Making

references to Monty Python skits in documentation is not only allowed,

it is encouraged!

\section{Where From Here \label{where}}

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 through 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.

\chapter{Using the Python Interpreter \label{using}}

\section{Invoking the Interpreter \label{invoking}}

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

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

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

typing the command

\begin{verbatim}

python

\end{verbatim}

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.,

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

Typing an EOF character (Control-D on \UNIX{}, Control-Z on DOS

or Windows) at the primary prompt causes the interpreter to exit with

a zero exit status. If that doesn't work, you can exit the

interpreter by typing the following commands: \samp{import sys;

sys.exit()}.

The interpreter's line-editing features usually aren't very

sophisticated. On \UNIX{}, whoever installed the interpreter may have

enabled support for the GNU readline library, which adds more

elaborate interactive editing and history features. 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; see Appendix A for an introduction to the

keys. If nothing appears to happen, or if \code{\^P} is echoed,

command line editing isn't available; you'll only be able to use

backspace to remove characters from the current line.

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 \emph{script} from

that file.

A third way of starting the interpreter is

\samp{python -c command [arg] ...}, which

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

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

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

\code{command} in its entirety with double quotes.

Note that there is a difference between \samp{python file} and

\samp{python <file}. In the latter case, input requests from the

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

satisfied from \emph{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 \code{-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 \label{argPassing}}

When known to the interpreter, the script name and additional

arguments thereafter are passed to the script in the variable

\code{sys.argv}, which is a list of strings. Its length is at least

one; when no script and no arguments are given, \code{sys.argv[0]} is

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

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

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

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

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

command to handle.

\subsection{Interactive Mode \label{interactive}}

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

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

with the \emph{primary prompt}, usually three greater-than signs

(\samp{>>> }); for continuation lines it prompts with the

\emph{secondary prompt},

by default three dots (\samp{... }).

The interpreter prints a welcome message stating its version number

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

\begin{verbatim}

python

Python 1.5.2b2 (#1, Feb 28 1999, 00:02:06) [GCC 2.8.1] on sunos5

Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam

>>>

\end{verbatim}

\section{The Interpreter and Its Environment \label{interp}}

\subsection{Error Handling \label{error}}

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 \code{except} clause in a

\code{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

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

\code{try} statement.

\subsection{Executable Python Scripts \label{scripts}}

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

executable, like shell scripts, by putting the line

\begin{verbatim}

#! /usr/bin/env python

\end{verbatim}

(assuming that the interpreter is on the user's \envvar{PATH}) at the

beginning of the script and giving the file an executable mode. The

\samp{\#!} must be the first two characters of the file. Note that

the hash, or pound, character, \character{\#}, is used to start a

comment in Python.

\subsection{The Interactive Startup File \label{startup}}

% XXX This should probably be dumped in an appendix, since most people

% don't use Python interactively in non-trivial ways.

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

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

commands. This is similar to the \file{.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 \file{/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 \code{sys.ps1} and \code{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.\ \samp{execfile('.pythonrc.py')}\indexii{.pythonrc.py}{file}. If

you want to use the startup file in a script, you must do this

explicitly in the script:

\begin{verbatim}

import os

if os.environ.get('PYTHONSTARTUP') \

and os.path.isfile(os.environ['PYTHONSTARTUP']):

execfile(os.environ['PYTHONSTARTUP'])

\end{verbatim}

\chapter{An Informal Introduction to Python \label{informal}}

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

presence or absence of prompts (\samp{>>> } and \samp{... }): 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.

Many of the examples in this manual, even those entered at the

interactive prompt, include comments. Comments in Python start with

the hash character, \character{\#}, and extend to the end of the

physical line. A comment may appear at the start of a line or

following whitespace or code, but not within a string literal. A hash

character within a string literal is just a hash character.

Some examples:

\begin{verbatim}

# this is the first comment

SPAM = 1 # and this is the second comment

# ... and now a third!

STRING = "# This is not a comment."

\end{verbatim}

\section{Using Python as a Calculator \label{calculator}}

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

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

\subsection{Numbers \label{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 \code{+}, \code{-}, \code{*} and \code{/}

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

can be used for grouping. For example:

\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}

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

variable. The value of an assignment is not written:

\begin{verbatim}

>>> width = 20

>>> height = 5*9

>>> width * height

900

\end{verbatim}

%

A value can be assigned to several variables simultaneously:

\begin{verbatim}

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

>>> x

0

>>> y

0

>>> z

0

\end{verbatim}

%

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

operands convert the integer operand to floating point:

\begin{verbatim}

>>> 4 * 2.5 / 3.3

3.0303030303

>>> 7.0 / 2

3.5

\end{verbatim}

%

Complex numbers are also supported; imaginary numbers are written with

a suffix of \samp{j} or \samp{J}. Complex numbers with a nonzero

real component are written as \samp{(\var{real}+\var{imag}j)}, or can

be created with the \samp{complex(\var{real}, \var{imag})} function.

\begin{verbatim}

>>> 1j * 1J

(-1+0j)

>>> 1j * complex(0,1)

(-1+0j)

>>> 3+1j*3

(3+3j)

>>> (3+1j)*3

(9+3j)

>>> (1+2j)/(1+1j)

(1.5+0.5j)

\end{verbatim}

%

Complex numbers are always represented as two floating point numbers,

the real and imaginary part. To extract these parts from a complex

number \var{z}, use \code{\var{z}.real} and \code{\var{z}.imag}.

\begin{verbatim}

>>> a=1.5+0.5j

>>> a.real

1.5

>>> a.imag

0.5

\end{verbatim}

%

The conversion functions to floating point and integer

(\function{float()}, \function{int()} and \function{long()}) don't

work for complex numbers --- there is no one correct way to convert a

complex number to a real number. Use \code{abs(\var{z})} to get its

magnitude (as a float) or \code{z.real} to get its real part.

\begin{verbatim}

>>> a=1.5+0.5j

>>> float(a)

Traceback (innermost last):

File "<stdin>", line 1, in ?

TypeError: can't convert complex to float; use e.g. abs(z)

>>> a.real

1.5

>>> abs(a)

1.58113883008

\end{verbatim}

%

In interactive mode, the last printed expression is assigned to the

variable \code{_}. This means that when you are using Python as a

desk calculator, it is somewhat easier to continue calculations, for

example:

\begin{verbatim}

>>> tax = 17.5 / 100

>>> price = 3.50

>>> price * tax

0.6125

>>> price + _

4.1125

>>> round(_, 2)

4.11

\end{verbatim}

This variable should be treated as read-only by the user. Don't

explicitly assign a value to it --- you would create an independent

local variable with the same name masking the built-in variable with

its magic behavior.

\subsection{Strings \label{strings}}

Besides numbers, Python can also manipulate strings, which can be

expressed in several ways. They can be enclosed in single quotes or

double quotes:

\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}

String literals can span multiple lines in several ways. Newlines can

be escaped with backslashes, e.g.:

\begin{verbatim}

hello = "This is a rather long string containing\n\

several lines of text just as you would do in C.\n\

Note that whitespace at the beginning of the line is\

significant.\n"

print hello

\end{verbatim}

which would print the following:

\begin{verbatim}

This is a rather long string containing

several lines of text just as you would do in C.

Note that whitespace at the beginning of the line is significant.

\end{verbatim}

Or, strings can be surrounded in a pair of matching triple-quotes:

\code{"""} or \code {'''}. End of lines do not need to be escaped

when using triple-quotes, but they will be included in the string.

\begin{verbatim}

print """

Usage: thingy [OPTIONS]

-h Display this usage message

-H hostname Hostname to connect to

"""

\end{verbatim}

produces the following output:

\begin{verbatim}

Usage: thingy [OPTIONS]

-h Display this usage message

-H hostname Hostname to connect to

\end{verbatim}

The interpreter prints the result of string operations in 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 \keyword{print} statement, described later, can be used

to write strings without quotes or escapes.)

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

operator, and repeated with \code{*}:

\begin{verbatim}

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

>>> word

'HelpA'

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

'<HelpAHelpAHelpAHelpAHelpA>'

\end{verbatim}

Two string literals next to each other are automatically concatenated;

the first line above could also have been written \samp{word = 'Help'

'A'}; this only works with two literals, not with arbitrary string

expressions:

\begin{verbatim}

>>> 'str' 'ing' # <- This is ok

'string'

>>> string.strip('str') + 'ing' # <- This is ok

'string'

>>> string.strip('str') 'ing' # <- This is invalid

File "<stdin>", line 1

string.strip('str') 'ing'

^

SyntaxError: invalid syntax

\end{verbatim}

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 \emph{slice notation}: two indices

separated by a colon.

\begin{verbatim}

>>> word[4]

'A'

>>> word[0:2]

'He'

>>> word[2:4]

'lp'

\end{verbatim}

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.

\begin{verbatim}

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

'He'

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

'lpA'

\end{verbatim}

Here's a useful invariant of slice operations: \code{s[:i] + s[i:]}

equals \code{s}.

\begin{verbatim}

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

'HelpA'

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

'HelpA'

\end{verbatim}

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.

\begin{verbatim}

>>> word[1:100]

'elpA'

>>> word[10:]

''

>>> word[2:1]

''

\end{verbatim}

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

For example:

\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}

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

the right!

\begin{verbatim}

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

'H'

\end{verbatim}

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

for single-element (non-slice) indices:

\begin{verbatim}

>>> word[-100:]

'HelpA'

>>> word[-10] # error

Traceback (innermost last):

File "<stdin>", line 1

IndexError: string index out of range

\end{verbatim}

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

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

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

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

\begin{verbatim}

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

| H | e | l | p | A |

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

0 1 2 3 4 5

-5 -4 -3 -2 -1

\end{verbatim}

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 \var{i} to \var{j} consists of all characters between

the edges labeled \var{i} and \var{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

\code{word[1:3]} is 2.

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

\begin{verbatim}

>>> s = 'supercalifragilisticexpialidocious'

>>> len(s)

34

\end{verbatim}

\subsection{Lists \label{lists}}

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

together other values. The most versatile is the \emph{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.

\begin{verbatim}

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

>>> a

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

\end{verbatim}

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

concatenated and so on:

\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}

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

individual elements of a list:

\begin{verbatim}

>>> a

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

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

>>> a

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

\end{verbatim}

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

of the list:

\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}

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

\begin{verbatim}

>>> len(a)

8

\end{verbatim}

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

for example:

\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}

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

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

\section{First Steps Towards Programming \label{firstSteps}}

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 \emph{Fibonacci} series as follows:

\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}

This example introduces several new features.

\begin{itemize}

\item

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

\code{a} and \code{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 \keyword{while} loop executes as long as the condition (here:

\code{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: \code{<}, \code{>}, \code{==}, \code{<=},

\code{>=} and \code{!=}.

\item

The \emph{body} of the loop is \emph{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 \keyword{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:

\begin{verbatim}

>>> i = 256*256

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

The value of i is 65536

\end{verbatim}

A trailing comma avoids the newline after the output:

\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}

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 \label{moreControl}}

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

the usual control flow statements known from other languages, with

some twists.

\section{\keyword{if} Statements \label{if}}

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

statement. For example:

\begin{verbatim}

>>> # [Code which sets 'x' to a value...]

>>> 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}

There can be zero or more \keyword{elif} parts, and the \keyword{else}

part is optional. The keyword `\keyword{elif}' is short for `else

if', and is useful to avoid excessive indentation. An

\keyword{if} \ldots\ \keyword{elif} \ldots\ \keyword{elif}

\ldots\ sequence is a substitute for the \emph{switch} or

% ^^^^

% Weird spacings happen here if the wrapping of the source text

% gets changed in the wrong way.

\emph{case} statements found in other languages.

\section{\keyword{for} Statements \label{for}}

The \keyword{for}\stindex{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 giving the user the ability to define both the iteration step and

halting condition (as C), Python's \keyword{for}\stindex{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):

% One suggestion was to give a real C example here, but that may only

% serve to confuse non-C programmers.

\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}

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:

\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}

\section{The \function{range()} Function \label{range}}

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

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

containing arithmetic progressions, e.g.:

\begin{verbatim}

>>> range(10)

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

\end{verbatim}

The given end point is never part of the generated list;

\code{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):

\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}

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

and \function{len()} as follows:

\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}

\section{\keyword{break} and \keyword{continue} Statements, and

\keyword{else} Clauses on Loops

\label{break}}

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

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

The \keyword{continue} statement, also borrowed from C, continues

with the next iteration of the loop.