and additional documentation.

The Python interpreter is easily extended with new functions and data

Python is also suitable as an extension language for customizable

applications.

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

Python in depth.

This tutorial does not attempt to be comprehensive and cover every

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

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

other data types (like sorted lists of file names) that are easy in

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

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

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

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.

It is also a handy desk calculator.

Python allows writing very compact and readable programs. Programs

programs, for several reasons:

\begin{itemize}

no variable or argument declarations are necessary.

\end{itemize}

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,

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

sys.exit()}.

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

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

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{/}

can be used for grouping. For example:

\bcode\begin{verbatim}

\end{verbatim}\ecode

variable. The value of an assignment is not written:

\bcode\begin{verbatim}

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

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

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

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

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

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

\section{For Statements}

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

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

\code{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):

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

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

next iteration of the loop.

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

\end{verbatim}\ecode

You might object that \code{fib} is not a function but a procedure. In

value. In fact, technically speaking, procedures do return a value,

albeit a rather boring one. This value is called \code{None} (it's a

built-in name). Writing the value \code{None} is normally suppressed by

\end{verbatim}\ecode

\section{Comparing Sequences and Other Types}