\documentclass{manual}

\title{Python Reference Manual}

\input{boilerplate}

\begin{document}

\input{copyright}

\begin{abstract}

Python is a simple, yet powerful, interpreted 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} (including Linux), the Apple Macintosh O.S.,

MS-DOS, MS-Windows 3.1, Windows NT, and OS/2.

This reference manual describes the syntax and ``core semantics'' of

the language. It is terse, but attempts to be exact and complete.

The semantics of non-essential built-in object types and of the

built-in functions and modules are described in the {\em Python

Library Reference}. For an informal introduction to the language, see

the {\em Python Tutorial}.

\end{abstract}

\include{ref1} % Introduction

\include{ref2} % Lexical analysis

\include{ref3} % Data model

\include{ref4} % Execution model

\include{ref5} % Expressions and conditions

\include{ref6} % Simple statements

\include{ref7} % Compound statements

\include{ref8} % Top-level components

\input{ref.ind}

\end{document}

\chapter{Introduction}

This reference manual describes the Python programming language.

It is not intended as a tutorial.

While I am trying to be as precise as possible, I chose to use English

rather than formal specifications for everything except syntax and

lexical analysis. This should make the document more understandable

to the average reader, but will leave room for ambiguities.

Consequently, if you were coming from Mars and tried to re-implement

Python from this document alone, you might have to guess things and in

fact you would probably end up implementing quite a different language.

On the other hand, if you are using

Python and wonder what the precise rules about a particular area of

the language are, you should definitely be able to find them here.

It is dangerous to add too many implementation details to a language

reference document --- the implementation may change, and other

implementations of the same language may work differently. On the

other hand, there is currently only one Python implementation, and

its particular quirks are sometimes worth being mentioned, especially

where the implementation imposes additional limitations. Therefore,

you'll find short ``implementation notes'' sprinkled throughout the

text.

Every Python implementation comes with a number of built-in and

standard modules. These are not documented here, but in the separate

{\em Python Library Reference} document. A few built-in modules are

mentioned when they interact in a significant way with the language

definition.

\section{Notation}

The descriptions of lexical analysis and syntax use a modified BNF

grammar notation. This uses the following style of definition:

\index{BNF}

\index{grammar}

\index{syntax}

\index{notation}

\begin{verbatim}

\end{verbatim}

The first line says that a \verb@name@ is an \verb@lc_letter@ followed by

a sequence of zero or more \verb@lc_letter@s and underscores. An

\verb@lc_letter@ in turn is any of the single characters `a' through `z'.

(This rule is actually adhered to for the names defined in lexical and

grammar rules in this document.)

Each rule begins with a name (which is the name defined by the rule)

and a colon. A vertical bar (\verb@|@) is used to separate

alternatives; it is the least binding operator in this notation. A

star (\verb@*@) means zero or more repetitions of the preceding item;

likewise, a plus (\verb@+@) means one or more repetitions, and a

phrase enclosed in square brackets (\verb@[ ]@) means zero or one

occurrences (in other words, the enclosed phrase is optional). The

\verb@*@ and \verb@+@ operators bind as tightly as possible;

parentheses are used for grouping. Literal strings are enclosed in

quotes. White space is only meaningful to separate tokens.

Rules are normally contained on a single line; rules with many

alternatives may be formatted alternatively with each line after the

first beginning with a vertical bar.

In lexical definitions (as the example above), two more conventions

are used: Two literal characters separated by three dots mean a choice

of any single character in the given (inclusive) range of \ASCII{}

characters. A phrase between angular brackets (\verb@<...>@) gives an

informal description of the symbol defined; e.g. this could be used

to describe the notion of `control character' if needed.

\index{lexical definitions}

\index{ASCII}

Even though the notation used is almost the same, there is a big

difference between the meaning of lexical and syntactic definitions:

a lexical definition operates on the individual characters of the

input source, while a syntax definition operates on the stream of

tokens generated by the lexical analysis. All uses of BNF in the next

chapter (``Lexical Analysis'') are lexical definitions; uses in

subsequent chapters are syntactic definitions.