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Merged revisions 69798 via svnmerge from svn+ssh://pythondev@svn.python.org/python/branches/py3k ................ r69798 | benjamin.peterson | 2009-02-19 22:02:38 -0600 (Thu, 19 Feb 2009) | 13 lines Merged revisions 69769,69776 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r69769 | georg.brandl | 2009-02-19 02:30:06 -0600 (Thu, 19 Feb 2009) | 1 line #5310, #3558: fix operator precedence table. ........ r69776 | georg.brandl | 2009-02-19 10:34:51 -0600 (Thu, 19 Feb 2009) | 2 lines #5317: update IronPython URL. ........ ................
author Benjamin Peterson <benjamin@python.org>
date Fri, 20 Feb 2009 04:05:32 +0000
parents 4ed16ca4d980
children 8df619259056
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1
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2 .. _introduction:
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3
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4 ************
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5 Introduction
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6 ************
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7
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8 This reference manual describes the Python programming language. It is not
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9 intended as a tutorial.
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10
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11 While I am trying to be as precise as possible, I chose to use English rather
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12 than formal specifications for everything except syntax and lexical analysis.
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13 This should make the document more understandable to the average reader, but
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14 will leave room for ambiguities. Consequently, if you were coming from Mars and
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15 tried to re-implement Python from this document alone, you might have to guess
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16 things and in fact you would probably end up implementing quite a different
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17 language. On the other hand, if you are using Python and wonder what the precise
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18 rules about a particular area of the language are, you should definitely be able
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19 to find them here. If you would like to see a more formal definition of the
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20 language, maybe you could volunteer your time --- or invent a cloning machine
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21 :-).
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22
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23 It is dangerous to add too many implementation details to a language reference
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24 document --- the implementation may change, and other implementations of the
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25 same language may work differently. On the other hand, CPython is the one
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26 Python implementation in widespread use (although alternate implementations
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27 continue to gain support), and its particular quirks are sometimes worth being
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28 mentioned, especially where the implementation imposes additional limitations.
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29 Therefore, you'll find short "implementation notes" sprinkled throughout the
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30 text.
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31
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32 Every Python implementation comes with a number of built-in and standard
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33 modules. These are documented in :ref:`library-index`. A few built-in modules
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34 are mentioned when they interact in a significant way with the language
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35 definition.
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36
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37
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38 .. _implementations:
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40 Alternate Implementations
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41 =========================
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42
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43 Though there is one Python implementation which is by far the most popular,
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44 there are some alternate implementations which are of particular interest to
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45 different audiences.
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46
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47 Known implementations include:
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48
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49 CPython
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50 This is the original and most-maintained implementation of Python, written in C.
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51 New language features generally appear here first.
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52
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53 Jython
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54 Python implemented in Java. This implementation can be used as a scripting
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55 language for Java applications, or can be used to create applications using the
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56 Java class libraries. It is also often used to create tests for Java libraries.
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57 More information can be found at `the Jython website <http://www.jython.org/>`_.
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59 Python for .NET
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60 This implementation actually uses the CPython implementation, but is a managed
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61 .NET application and makes .NET libraries available. It was created by Brian
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62 Lloyd. For more information, see the `Python for .NET home page
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63 <http://pythonnet.sourceforge.net>`_.
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64
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65 IronPython
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66 An alternate Python for .NET. Unlike Python.NET, this is a complete Python
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67 implementation that generates IL, and compiles Python code directly to .NET
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68 assemblies. It was created by Jim Hugunin, the original creator of Jython. For
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69 more information, see `the IronPython website <http://www.ironpython.com/>`_.
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71 PyPy
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72 An implementation of Python written in Python; even the bytecode interpreter is
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73 written in Python. This is executed using CPython as the underlying
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74 interpreter. One of the goals of the project is to encourage experimentation
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75 with the language itself by making it easier to modify the interpreter (since it
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76 is written in Python). Additional information is available on `the PyPy
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77 project's home page <http://codespeak.net/pypy/>`_.
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78
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79 Each of these implementations varies in some way from the language as documented
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80 in this manual, or introduces specific information beyond what's covered in the
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81 standard Python documentation. Please refer to the implementation-specific
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82 documentation to determine what else you need to know about the specific
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83 implementation you're using.
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84
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85
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86 .. _notation:
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87
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88 Notation
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89 ========
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90
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91 .. index:: BNF, grammar, syntax, notation
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92
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93 The descriptions of lexical analysis and syntax use a modified BNF grammar
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94 notation. This uses the following style of definition:
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95
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96 .. productionlist:: *
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97 name: `lc_letter` (`lc_letter` | "_")*
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98 lc_letter: "a"..."z"
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99
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100 The first line says that a ``name`` is an ``lc_letter`` followed by a sequence
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101 of zero or more ``lc_letter``\ s and underscores. An ``lc_letter`` in turn is
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102 any of the single characters ``'a'`` through ``'z'``. (This rule is actually
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103 adhered to for the names defined in lexical and grammar rules in this document.)
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104
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105 Each rule begins with a name (which is the name defined by the rule) and
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106 ``::=``. A vertical bar (``|``) is used to separate alternatives; it is the
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107 least binding operator in this notation. A star (``*``) means zero or more
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108 repetitions of the preceding item; likewise, a plus (``+``) means one or more
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109 repetitions, and a phrase enclosed in square brackets (``[ ]``) means zero or
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110 one occurrences (in other words, the enclosed phrase is optional). The ``*``
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111 and ``+`` operators bind as tightly as possible; parentheses are used for
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112 grouping. Literal strings are enclosed in quotes. White space is only
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113 meaningful to separate tokens. Rules are normally contained on a single line;
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114 rules with many alternatives may be formatted alternatively with each line after
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115 the first beginning with a vertical bar.
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116
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117 .. index:: lexical definitions, ASCII
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118
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119 In lexical definitions (as the example above), two more conventions are used:
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120 Two literal characters separated by three dots mean a choice of any single
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121 character in the given (inclusive) range of ASCII characters. A phrase between
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122 angular brackets (``<...>``) gives an informal description of the symbol
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123 defined; e.g., this could be used to describe the notion of 'control character'
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124 if needed.
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125
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126 Even though the notation used is almost the same, there is a big difference
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127 between the meaning of lexical and syntactic definitions: a lexical definition
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128 operates on the individual characters of the input source, while a syntax
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129 definition operates on the stream of tokens generated by the lexical analysis.
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130 All uses of BNF in the next chapter ("Lexical Analysis") are lexical
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131 definitions; uses in subsequent chapters are syntactic definitions.
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132