.. _introduction:

************

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. If you would like to see a more formal definition of the

language, maybe you could volunteer your time --- or invent a cloning machine

:-).

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 in widespread use (although alternate implementations

exist), 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 documented in :ref:`library-index`.  A few built-in modules

are mentioned when they interact in a significant way with the language

definition.

.. _implementations:

Alternate Implementations

=========================

Though there is one Python implementation which is by far the most popular,

there are some alternate implementations which are of particular interest to

different audiences.

Known implementations include:

CPython

   This is the original and most-maintained implementation of Python, written in C.

   New language features generally appear here first.

Jython

   Python implemented in Java.  This implementation can be used as a scripting

   language for Java applications, or can be used to create applications using the

   Java class libraries.  It is also often used to create tests for Java libraries.

   More information can be found at `the Jython website <http://www.jython.org/>`_.

Python for .NET

   This implementation actually uses the CPython implementation, but is a managed

   .NET application and makes .NET libraries available.  It was created by Brian

   Lloyd.  For more information, see the `Python for .NET home page

   <http://pythonnet.sourceforge.net>`_.

IronPython

   An alternate Python for .NET.  Unlike Python.NET, this is a complete Python

   implementation that generates IL, and compiles Python code directly to .NET

   assemblies.  It was created by Jim Hugunin, the original creator of Jython.  For

   more information, see `the IronPython website

   <http://workspaces.gotdotnet.com/ironpython>`_.

PyPy

   An implementation of Python written in Python; even the bytecode interpreter is

   written in Python.  This is executed using CPython as the underlying

   interpreter.  One of the goals of the project is to encourage experimentation

   with the language itself by making it easier to modify the interpreter (since it

   is written in Python).  Additional information is available on `the PyPy

   project's home page <http://codespeak.net/pypy/>`_.

Each of these implementations varies in some way from the language as documented

in this manual, or introduces specific information beyond what's covered in the

standard Python documentation.  Please refer to the implementation-specific

documentation to determine what else you need to know about the specific

implementation you're using.

.. _notation:

Notation

========

.. index::

   single: BNF

   single: grammar

   single: syntax

   single: notation

The descriptions of lexical analysis and syntax use a modified BNF grammar

notation.  This uses the following style of definition:

.. productionlist:: *

   name: `lc_letter` (`lc_letter` | "_")*

   lc_letter: "a"..."z"

The first line says that a ``name`` is an ``lc_letter`` followed by a sequence

of zero or more ``lc_letter``\ s and underscores.  An ``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 vertical bar (``|``) is used to separate alternatives; it is the

least binding operator in this notation.  A star (``*``) means zero or more

repetitions of the preceding item; likewise, a plus (``+``) means one or more

repetitions, and a phrase enclosed in square brackets (``[ ]``) means zero or

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

and ``+`` 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.

.. index::

   single: lexical definitions

   single: ASCII@ASCII

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 (``<...>``) gives an informal description of the symbol

defined; e.g., this could be used to describe the notion of 'control character'

if needed.

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.