8
This reference manual describes the Python programming language. It is not
9
intended as a tutorial.
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While I am trying to be as precise as possible, I chose to use English rather
12
than formal specifications for everything except syntax and lexical analysis.
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This should make the document more understandable to the average reader, but
14
will leave room for ambiguities. Consequently, if you were coming from Mars and
15
tried to re-implement Python from this document alone, you might have to guess
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things and in fact you would probably end up implementing quite a different
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language. On the other hand, if you are using Python and wonder what the precise
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rules about a particular area of the language are, you should definitely be able
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to find them here. If you would like to see a more formal definition of the
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language, maybe you could volunteer your time --- or invent a cloning machine
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It is dangerous to add too many implementation details to a language reference
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document --- the implementation may change, and other implementations of the
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same language may work differently. On the other hand, CPython is the one
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Python implementation in widespread use (although alternate implementations
27
continue to gain support), and its particular quirks are sometimes worth being
28
mentioned, especially where the implementation imposes additional limitations.
29
Therefore, you'll find short "implementation notes" sprinkled throughout the
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Every Python implementation comes with a number of built-in and standard
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modules. These are documented in :ref:`library-index`. A few built-in modules
34
are mentioned when they interact in a significant way with the language
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Alternate Implementations
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=========================
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Though there is one Python implementation which is by far the most popular,
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there are some alternate implementations which are of particular interest to
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Known implementations include:
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This is the original and most-maintained implementation of Python, written in C.
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New language features generally appear here first.
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Python implemented in Java. This implementation can be used as a scripting
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language for Java applications, or can be used to create applications using the
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Java class libraries. It is also often used to create tests for Java libraries.
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More information can be found at `the Jython website <http://www.jython.org/>`_.
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This implementation actually uses the CPython implementation, but is a managed
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.NET application and makes .NET libraries available. It was created by Brian
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Lloyd. For more information, see the `Python for .NET home page
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<http://pythonnet.sourceforge.net>`_.
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An alternate Python for .NET. Unlike Python.NET, this is a complete Python
67
implementation that generates IL, and compiles Python code directly to .NET
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assemblies. It was created by Jim Hugunin, the original creator of Jython. For
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more information, see `the IronPython website <http://www.ironpython.com/>`_.
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An implementation of Python written in Python; even the bytecode interpreter is
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written in Python. This is executed using CPython as the underlying
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interpreter. One of the goals of the project is to encourage experimentation
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with the language itself by making it easier to modify the interpreter (since it
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is written in Python). Additional information is available on `the PyPy
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project's home page <http://codespeak.net/pypy/>`_.
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Each of these implementations varies in some way from the language as documented
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in this manual, or introduces specific information beyond what's covered in the
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standard Python documentation. Please refer to the implementation-specific
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documentation to determine what else you need to know about the specific
83
implementation you're using.
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.. index:: BNF, grammar, syntax, notation
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The descriptions of lexical analysis and syntax use a modified BNF grammar
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notation. This uses the following style of definition:
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name: `lc_letter` (`lc_letter` | "_")*
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The first line says that a ``name`` is an ``lc_letter`` followed by a sequence
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of zero or more ``lc_letter``\ s and underscores. An ``lc_letter`` in turn is
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any of the single characters ``'a'`` through ``'z'``. (This rule is actually
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adhered to for the names defined in lexical and grammar rules in this document.)
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Each rule begins with a name (which is the name defined by the rule) and
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``::=``. A vertical bar (``|``) is used to separate alternatives; it is the
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least binding operator in this notation. A star (``*``) means zero or more
108
repetitions of the preceding item; likewise, a plus (``+``) means one or more
109
repetitions, and a phrase enclosed in square brackets (``[ ]``) means zero or
110
one occurrences (in other words, the enclosed phrase is optional). The ``*``
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and ``+`` operators bind as tightly as possible; parentheses are used for
112
grouping. Literal strings are enclosed in quotes. White space is only
113
meaningful to separate tokens. Rules are normally contained on a single line;
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rules with many alternatives may be formatted alternatively with each line after
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the first beginning with a vertical bar.
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.. index:: lexical definitions, ASCII
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In lexical definitions (as the example above), two more conventions are used:
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Two literal characters separated by three dots mean a choice of any single
121
character in the given (inclusive) range of ASCII characters. A phrase between
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angular brackets (``<...>``) gives an informal description of the symbol
123
defined; e.g., this could be used to describe the notion of 'control character'
126
Even though the notation used is almost the same, there is a big difference
127
between the meaning of lexical and syntactic definitions: a lexical definition
128
operates on the individual characters of the input source, while a syntax
129
definition operates on the stream of tokens generated by the lexical analysis.
130
All uses of BNF in the next chapter ("Lexical Analysis") are lexical
131
definitions; uses in subsequent chapters are syntactic definitions.