1
2.. _introduction:
3
4************
5Introduction
6************
7
8This reference manual describes the Python programming language. It is not
9intended as a tutorial.
10
11While I am trying to be as precise as possible, I chose to use English rather
12than formal specifications for everything except syntax and lexical analysis.
13This should make the document more understandable to the average reader, but
14will leave room for ambiguities. Consequently, if you were coming from Mars and
15tried to re-implement Python from this document alone, you might have to guess
16things and in fact you would probably end up implementing quite a different
17language. On the other hand, if you are using Python and wonder what the precise
18rules about a particular area of the language are, you should definitely be able
19to find them here. If you would like to see a more formal definition of the
20language, maybe you could volunteer your time --- or invent a cloning machine
21:-).
22
23It is dangerous to add too many implementation details to a language reference
24document --- the implementation may change, and other implementations of the
25same language may work differently.  On the other hand, there is currently only
26one Python implementation in widespread use (although alternate implementations
27exist), and its particular quirks are sometimes worth being mentioned,
28especially where the implementation imposes additional limitations.  Therefore,
29you'll find short "implementation notes" sprinkled throughout the text.
30
31Every Python implementation comes with a number of built-in and standard
32modules.  These are documented in :ref:`library-index`.  A few built-in modules
33are mentioned when they interact in a significant way with the language
34definition.
35
36
37.. _implementations:
38
39Alternate Implementations
40=========================
41
42Though there is one Python implementation which is by far the most popular,
43there are some alternate implementations which are of particular interest to
44different audiences.
45
46Known implementations include:
47
48CPython
49   This is the original and most-maintained implementation of Python, written in C.
50   New language features generally appear here first.
51
52Jython
53   Python implemented in Java.  This implementation can be used as a scripting
54   language for Java applications, or can be used to create applications using the
55   Java class libraries.  It is also often used to create tests for Java libraries.
56   More information can be found at `the Jython website <http://www.jython.org/>`_.
57
58Python for .NET
59   This implementation actually uses the CPython implementation, but is a managed
60   .NET application and makes .NET libraries available.  It was created by Brian
61   Lloyd.  For more information, see the `Python for .NET home page
62   <https://pythonnet.github.io/>`_.
63
64IronPython
65   An alternate Python for .NET.  Unlike Python.NET, this is a complete Python
66   implementation that generates IL, and compiles Python code directly to .NET
67   assemblies.  It was created by Jim Hugunin, the original creator of Jython.  For
68   more information, see `the IronPython website <http://ironpython.net/>`_.
69
70PyPy
71   An implementation of Python written completely in Python. It supports several
72   advanced features not found in other implementations like stackless support
73   and a Just in Time compiler. One of the goals of the project is to encourage
74   experimentation with the language itself by making it easier to modify the
75   interpreter (since it is written in Python).  Additional information is
76   available on `the PyPy project's home page <http://pypy.org/>`_.
77
78Each of these implementations varies in some way from the language as documented
79in this manual, or introduces specific information beyond what's covered in the
80standard Python documentation.  Please refer to the implementation-specific
81documentation to determine what else you need to know about the specific
82implementation you're using.
83
84
85.. _notation:
86
87Notation
88========
89
90.. index::
91   single: BNF
92   single: grammar
93   single: syntax
94   single: notation
95
96The descriptions of lexical analysis and syntax use a modified BNF grammar
97notation.  This uses the following style of definition:
98
99.. productionlist:: *
100   name: `lc_letter` (`lc_letter` | "_")*
101   lc_letter: "a"..."z"
102
103The first line says that a ``name`` is an ``lc_letter`` followed by a sequence
104of zero or more ``lc_letter``\ s and underscores.  An ``lc_letter`` in turn is
105any of the single characters ``'a'`` through ``'z'``.  (This rule is actually
106adhered to for the names defined in lexical and grammar rules in this document.)
107
108Each rule begins with a name (which is the name defined by the rule) and
109``::=``.  A vertical bar (``|``) is used to separate alternatives; it is the
110least binding operator in this notation.  A star (``*``) means zero or more
111repetitions of the preceding item; likewise, a plus (``+``) means one or more
112repetitions, and a phrase enclosed in square brackets (``[ ]``) means zero or
113one occurrences (in other words, the enclosed phrase is optional).  The ``*``
114and ``+`` operators bind as tightly as possible; parentheses are used for
115grouping.  Literal strings are enclosed in quotes.  White space is only
116meaningful to separate tokens. Rules are normally contained on a single line;
117rules with many alternatives may be formatted alternatively with each line after
118the first beginning with a vertical bar.
119
120.. index::
121   single: lexical definitions
122   single: ASCII@ASCII
123
124In lexical definitions (as the example above), two more conventions are used:
125Two literal characters separated by three dots mean a choice of any single
126character in the given (inclusive) range of ASCII characters.  A phrase between
127angular brackets (``<...>``) gives an informal description of the symbol
128defined; e.g., this could be used to describe the notion of 'control character'
129if needed.
130
131Even though the notation used is almost the same, there is a big difference
132between the meaning of lexical and syntactic definitions: a lexical definition
133operates on the individual characters of the input source, while a syntax
134definition operates on the stream of tokens generated by the lexical analysis.
135All uses of BNF in the next chapter ("Lexical Analysis") are lexical
136definitions; uses in subsequent chapters are syntactic definitions.
137
138