cpython/Doc/reference/introduction.rst

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.. _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, CPython is the one
Python implementation in widespread use (although alternate implementations
continue to gain support), 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.
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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
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Lloyd. For more information, see the `Python for .NET home page
<https://pythonnet.github.io/>`_.
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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://ironpython.net/>`_.
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PyPy
Merged revisions 80605-80609,80642-80646,80651-80652,80674,80684-80686,80748,80852,80854,80870,80872-80873,80907,80915-80916,80951-80952,80976-80977,80985,81038-81040,81042,81053,81070,81104-81105,81114,81125,81245,81285,81402,81463,81516,81562-81563,81567,81593,81635,81680-81681,81684,81801,81888,81931-81933,81939-81942,81963,81984,81991,82120,82188,82264-82267 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r80605 | andrew.kuchling | 2010-04-28 19:22:16 -0500 (Wed, 28 Apr 2010) | 1 line Add various items ........ r80606 | andrew.kuchling | 2010-04-28 20:44:30 -0500 (Wed, 28 Apr 2010) | 6 lines Fix doubled 'the'. Markup fixes to use :exc:, :option: in a few places. (Glitch: unittest.main's -c ends up a link to the Python interpreter's -c option. Should we skip using :option: for that switch, or disable the auto-linking somehow?) ........ r80607 | andrew.kuchling | 2010-04-28 20:45:41 -0500 (Wed, 28 Apr 2010) | 1 line Add various unittest items ........ r80608 | benjamin.peterson | 2010-04-28 22:18:05 -0500 (Wed, 28 Apr 2010) | 1 line update pypy description ........ r80609 | benjamin.peterson | 2010-04-28 22:30:59 -0500 (Wed, 28 Apr 2010) | 1 line update pypy url ........ r80642 | andrew.kuchling | 2010-04-29 19:49:09 -0500 (Thu, 29 Apr 2010) | 1 line Always add space after RFC; reword paragraph ........ r80643 | andrew.kuchling | 2010-04-29 19:52:31 -0500 (Thu, 29 Apr 2010) | 6 lines Reword paragraph to make its meaning clearer. Antoine Pitrou: is my version of the paragraph still correct? R. David Murray: is this more understandable than the previous version? ........ r80644 | andrew.kuchling | 2010-04-29 20:02:15 -0500 (Thu, 29 Apr 2010) | 1 line Fix typos ........ r80645 | andrew.kuchling | 2010-04-29 20:32:47 -0500 (Thu, 29 Apr 2010) | 1 line Markup fix; clarify by adding 'in that order' ........ r80646 | andrew.kuchling | 2010-04-29 20:33:40 -0500 (Thu, 29 Apr 2010) | 1 line Add various items; rearrange unittest section a bit ........ r80651 | andrew.kuchling | 2010-04-30 08:46:55 -0500 (Fri, 30 Apr 2010) | 1 line Minor grammar re-wording ........ r80652 | andrew.kuchling | 2010-04-30 08:47:34 -0500 (Fri, 30 Apr 2010) | 1 line Add item ........ r80674 | andrew.kuchling | 2010-04-30 20:19:16 -0500 (Fri, 30 Apr 2010) | 1 line Add various items ........ r80684 | andrew.kuchling | 2010-05-01 07:05:52 -0500 (Sat, 01 May 2010) | 1 line Minor grammar fix ........ r80685 | andrew.kuchling | 2010-05-01 07:06:51 -0500 (Sat, 01 May 2010) | 1 line Describe memoryview ........ r80686 | antoine.pitrou | 2010-05-01 07:16:39 -0500 (Sat, 01 May 2010) | 4 lines Fix attribution. Travis didn't do much and he did a bad work. (yes, this is a sensitive subject, sorry) ........ r80748 | andrew.kuchling | 2010-05-03 20:24:22 -0500 (Mon, 03 May 2010) | 1 line Add some more items; the urlparse change is added twice ........ r80852 | andrew.kuchling | 2010-05-05 20:09:47 -0500 (Wed, 05 May 2010) | 1 line Reword paragraph; fix filename, which should be pyconfig.h ........ r80854 | andrew.kuchling | 2010-05-05 20:10:56 -0500 (Wed, 05 May 2010) | 1 line Add various items ........ r80870 | andrew.kuchling | 2010-05-06 09:14:09 -0500 (Thu, 06 May 2010) | 1 line Describe ElementTree 1.3; rearrange new-module sections; describe dict views as sets; small edits and items ........ r80872 | andrew.kuchling | 2010-05-06 12:21:59 -0500 (Thu, 06 May 2010) | 1 line Add 2 items; record ideas for two initial sections; clarify wording ........ r80873 | andrew.kuchling | 2010-05-06 12:27:57 -0500 (Thu, 06 May 2010) | 1 line Change section title; point to unittest2 ........ r80907 | andrew.kuchling | 2010-05-06 20:45:14 -0500 (Thu, 06 May 2010) | 1 line Add a new section on the development plan; add an item ........ r80915 | antoine.pitrou | 2010-05-07 05:15:51 -0500 (Fri, 07 May 2010) | 3 lines Fix some markup and a class name. Also, wrap a long line. ........ r80916 | andrew.kuchling | 2010-05-07 06:30:47 -0500 (Fri, 07 May 2010) | 1 line Re-word text ........ r80951 | andrew.kuchling | 2010-05-07 20:15:26 -0500 (Fri, 07 May 2010) | 1 line Add two items ........ r80952 | andrew.kuchling | 2010-05-07 20:35:55 -0500 (Fri, 07 May 2010) | 1 line Get accents correct ........ r80976 | andrew.kuchling | 2010-05-08 08:28:03 -0500 (Sat, 08 May 2010) | 1 line Add logging.dictConfig example; give up on writing a Ttk example ........ r80977 | andrew.kuchling | 2010-05-08 08:29:46 -0500 (Sat, 08 May 2010) | 1 line Markup fixes ........ r80985 | andrew.kuchling | 2010-05-08 10:39:46 -0500 (Sat, 08 May 2010) | 7 lines Write summary of the 2.7 release; rewrite the future section some more; mention PYTHONWARNINGS env. var; tweak some examples for readability. And with this commit, the "What's New" is done... except for a complete read-through to polish the text, and fixing any reported errors, but those tasks can easily wait until after beta2. ........ r81038 | benjamin.peterson | 2010-05-09 16:09:40 -0500 (Sun, 09 May 2010) | 1 line finish clause ........ r81039 | andrew.kuchling | 2010-05-10 09:18:27 -0500 (Mon, 10 May 2010) | 1 line Markup fix; re-word a sentence ........ r81040 | andrew.kuchling | 2010-05-10 09:20:12 -0500 (Mon, 10 May 2010) | 1 line Use title case ........ r81042 | andrew.kuchling | 2010-05-10 10:03:35 -0500 (Mon, 10 May 2010) | 1 line Link to unittest2 article ........ r81053 | florent.xicluna | 2010-05-10 14:59:22 -0500 (Mon, 10 May 2010) | 2 lines Add a link on maketrans(). ........ r81070 | andrew.kuchling | 2010-05-10 18:13:41 -0500 (Mon, 10 May 2010) | 1 line Fix typo ........ r81104 | andrew.kuchling | 2010-05-11 19:38:44 -0500 (Tue, 11 May 2010) | 1 line Revision pass: lots of edits, typo fixes, rearrangements ........ r81105 | andrew.kuchling | 2010-05-11 19:40:47 -0500 (Tue, 11 May 2010) | 1 line Let's call this done ........ r81114 | andrew.kuchling | 2010-05-12 08:56:07 -0500 (Wed, 12 May 2010) | 1 line Grammar fix ........ r81125 | andrew.kuchling | 2010-05-12 13:56:48 -0500 (Wed, 12 May 2010) | 1 line #8696: add documentation for logging.config.dictConfig (PEP 391) ........ r81245 | andrew.kuchling | 2010-05-16 18:31:16 -0500 (Sun, 16 May 2010) | 1 line Add cross-reference to later section ........ r81285 | vinay.sajip | 2010-05-18 03:16:27 -0500 (Tue, 18 May 2010) | 1 line Fixed minor typo in ReST markup. ........ r81402 | vinay.sajip | 2010-05-21 12:41:34 -0500 (Fri, 21 May 2010) | 1 line Updated logging documentation with more dictConfig information. ........ r81463 | georg.brandl | 2010-05-22 03:17:23 -0500 (Sat, 22 May 2010) | 1 line #8785: less confusing description of regex.find*. ........ r81516 | andrew.kuchling | 2010-05-25 08:34:08 -0500 (Tue, 25 May 2010) | 1 line Add three items ........ r81562 | andrew.kuchling | 2010-05-27 08:22:53 -0500 (Thu, 27 May 2010) | 1 line Rewrite wxWidgets section ........ r81563 | andrew.kuchling | 2010-05-27 08:30:09 -0500 (Thu, 27 May 2010) | 1 line Remove top-level 'General Questions' section, pushing up the questions it contains ........ r81567 | andrew.kuchling | 2010-05-27 16:29:59 -0500 (Thu, 27 May 2010) | 1 line Add item ........ r81593 | georg.brandl | 2010-05-29 03:46:18 -0500 (Sat, 29 May 2010) | 1 line #8616: add new turtle demo "nim". ........ r81635 | georg.brandl | 2010-06-01 02:25:23 -0500 (Tue, 01 Jun 2010) | 1 line Put docs for RegexObject.search() before RegexObject.match() to mirror re.search() and re.match() order. ........ r81680 | vinay.sajip | 2010-06-03 17:34:42 -0500 (Thu, 03 Jun 2010) | 1 line Issue #8890: Documentation changed to avoid reference to temporary files. ........ r81681 | sean.reifschneider | 2010-06-03 20:51:26 -0500 (Thu, 03 Jun 2010) | 2 lines Issue8810: Clearing up docstring for tzinfo.utcoffset. ........ r81684 | vinay.sajip | 2010-06-04 08:41:02 -0500 (Fri, 04 Jun 2010) | 1 line Issue #8890: Documentation changed to avoid reference to temporary files - other cases covered. ........ r81801 | andrew.kuchling | 2010-06-07 08:38:40 -0500 (Mon, 07 Jun 2010) | 1 line #8875: Remove duplicated paragraph ........ r81888 | andrew.kuchling | 2010-06-10 20:54:58 -0500 (Thu, 10 Jun 2010) | 1 line Add a few more items ........ r81931 | georg.brandl | 2010-06-12 01:26:54 -0500 (Sat, 12 Jun 2010) | 1 line Fix punctuation. ........ r81932 | georg.brandl | 2010-06-12 01:28:58 -0500 (Sat, 12 Jun 2010) | 1 line Document that an existing directory raises in mkdir(). ........ r81933 | georg.brandl | 2010-06-12 01:45:33 -0500 (Sat, 12 Jun 2010) | 1 line Update version in README. ........ r81939 | georg.brandl | 2010-06-12 04:45:01 -0500 (Sat, 12 Jun 2010) | 1 line Use newer toctree syntax. ........ r81940 | georg.brandl | 2010-06-12 04:45:28 -0500 (Sat, 12 Jun 2010) | 1 line Add document on how to build. ........ r81941 | georg.brandl | 2010-06-12 04:45:58 -0500 (Sat, 12 Jun 2010) | 1 line Fix gratuitous indentation. ........ r81942 | georg.brandl | 2010-06-12 04:46:03 -0500 (Sat, 12 Jun 2010) | 1 line Update README. ........ r81963 | andrew.kuchling | 2010-06-12 15:00:55 -0500 (Sat, 12 Jun 2010) | 1 line Grammar fix ........ r81984 | georg.brandl | 2010-06-14 10:58:39 -0500 (Mon, 14 Jun 2010) | 1 line #8993: fix reference. ........ r81991 | andrew.kuchling | 2010-06-14 19:38:58 -0500 (Mon, 14 Jun 2010) | 1 line Add another bunch of items ........ r82120 | andrew.kuchling | 2010-06-20 16:45:45 -0500 (Sun, 20 Jun 2010) | 1 line Note that Python 3.x isn't covered; add forward ref. for UTF-8; note error in 2.5 and up ........ r82188 | benjamin.peterson | 2010-06-23 19:02:46 -0500 (Wed, 23 Jun 2010) | 1 line remove reverted changed ........ r82264 | georg.brandl | 2010-06-27 05:47:47 -0500 (Sun, 27 Jun 2010) | 1 line Confusing punctuation. ........ r82265 | georg.brandl | 2010-06-27 05:49:23 -0500 (Sun, 27 Jun 2010) | 1 line Use designated syntax for optional grammar element. ........ r82266 | georg.brandl | 2010-06-27 05:51:44 -0500 (Sun, 27 Jun 2010) | 1 line Fix URL. ........ r82267 | georg.brandl | 2010-06-27 05:55:38 -0500 (Sun, 27 Jun 2010) | 1 line Two typos. ........
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An implementation of Python written completely in Python. It supports several
advanced features not found in other implementations like stackless support
and a Just in Time compiler. 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://pypy.org/>`_.
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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:: BNF, grammar, syntax, notation
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The descriptions of lexical analysis and syntax use a modified BNF grammar
notation. This uses the following style of definition:
[3.9] bpo-40204: Allow pre-Sphinx 3 syntax in the doc (GH-21844) (GH-21901) * bpo-40204: Allow pre-Sphinx 3 syntax in the doc (GH-21844) Enable Sphinx 3.2 "c_allow_pre_v3" option and disable the c_warn_on_allowed_pre_v3 option to make the documentation compatible with Sphinx 2 and Sphinx 3. (cherry picked from commit 423e77d6de497931585d1883805a9e3fa4096b0b) * bpo-40204: Fix Sphinx sytanx in howto/instrumentation.rst (GH-21858) Use generic '.. object::' to declare markers, rather than abusing '.. c:function::' which fails on Sphinx 3. (cherry picked from commit 43577c01a2ab49122db696e9eaec6cb31d11cc81) * bpo-40204: Fix duplicates in the documentation (GH-21857) Fix two Sphinx 3 issues: Doc/c-api/buffer.rst:304: WARNING: Duplicate C declaration, also defined in 'c-api/buffer'. Declaration is 'PyBUF_ND'. Doc/c-api/unicode.rst:1603: WARNING: Duplicate C declaration, also defined in 'c-api/unicode'. Declaration is 'PyObject* PyUnicode_Translate(PyObject *str, PyObject *table, const char *errors)'. (cherry picked from commit 46d10b1237c67ff8347f533eda6a5468d098f7eb) * bpo-40204: Add :noindex: in the documentation (GH-21859) Add :noindex: to duplicated documentation to fix "duplicate object description" errors. For example, fix this Sphinx 3 issue: Doc/library/configparser.rst:1146: WARNING: duplicate object description of configparser.ConfigParser.optionxform, other instance in library/configparser, use :noindex: for one of them (cherry picked from commit d3ded080482beae578faa704b13534a62d066f9f) * bpo-40204, doc: Fix syntax of C variables (GH-21846) For example, fix the following Sphinx 3 errors: Doc/c-api/buffer.rst:102: WARNING: Error in declarator or parameters Invalid C declaration: Expected identifier in nested name. [error at 5] void \*obj -----^ Doc/c-api/arg.rst:130: WARNING: Unparseable C cross-reference: 'PyObject*' Invalid C declaration: Expected end of definition. [error at 8] PyObject* --------^ The modified documentation is compatible with Sphinx 2 and Sphinx 3. (cherry picked from commit 474652fe9346382dbf793f20b671eb74668bebde) * bpo-40204: Fix reference to terms in the doc (GH-21865) Sphinx 3 requires to refer to terms with the exact case. For example, fix the Sphinx 3 warning: Doc/library/pkgutil.rst:71: WARNING: term Loader not found in case sensitive match.made a reference to loader instead. (cherry picked from commit bb0b08540cc93e56f3f1bde1b39ce086d9e35fe1) * bpo-40204: Fix duplicated productionlist names in the doc (GH-21900) Sphinx 3 disallows having more than one productionlist markup with the same name. Simply remove names in this case, since names are not shown anyway. For example, fix the Sphinx 3 warning: Doc/reference/introduction.rst:96: duplicate token description of *:name, other instance in reference/expressions (cherry picked from commit 1abeda80f760134b4233608e2c288790f955b95a)
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.. productionlist::
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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:: lexical definitions, ASCII
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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.