cpython/Doc/library/constants.rst

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.. _built-in-consts:
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Built-in Constants
==================
A small number of constants live in the built-in namespace. They are:
.. data:: False
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The false value of the :class:`bool` type. Assignments to ``False``
are illegal and raise a :exc:`SyntaxError`.
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.. data:: True
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The true value of the :class:`bool` type. Assignments to ``True``
are illegal and raise a :exc:`SyntaxError`.
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.. data:: None
The sole value of the type ``NoneType``. ``None`` is frequently used to
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represent the absence of a value, as when default arguments are not passed to a
function. Assignments to ``None`` are illegal and raise a :exc:`SyntaxError`.
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.. data:: NotImplemented
Special value which should be returned by the binary special methods
(e.g. :meth:`__eq__`, :meth:`__lt__`, :meth:`__add__`, :meth:`__rsub__`,
etc.) to indicate that the operation is not implemented with respect to
the other type; may be returned by the in-place binary special methods
(e.g. :meth:`__imul__`, :meth:`__iand__`, etc.) for the same purpose.
Its truth value is true.
.. note::
When a binary (or in-place) method returns ``NotImplemented`` the
interpreter will try the reflected operation on the other type (or some
other fallback, depending on the operator). If all attempts return
``NotImplemented``, the interpreter will raise an appropriate exception.
Incorrectly returning ``NotImplemented`` will result in a misleading
error message or the ``NotImplemented`` value being returned to Python code.
See :ref:`implementing-the-arithmetic-operations` for examples.
.. note::
``NotImplentedError`` and ``NotImplemented`` are not interchangeable,
even though they have similar names and purposes.
See :exc:`NotImplementedError` for details on when to use it.
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.. data:: Ellipsis
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The same as ``...``. Special value used mostly in conjunction with extended
slicing syntax for user-defined container data types.
.. data:: __debug__
This constant is true if Python was not started with an :option:`-O` option.
See also the :keyword:`assert` statement.
Merged revisions 60124-60142 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r60131 | georg.brandl | 2008-01-20 12:13:29 +0100 (Sun, 20 Jan 2008) | 3 lines #1351692: in pprint, always call format() for dict and list items to enable custom formatting of contents via subclassing PrettyPrinter. ........ r60133 | georg.brandl | 2008-01-20 12:43:03 +0100 (Sun, 20 Jan 2008) | 2 lines #1178141: add addinfourl.code to get http status code from urllib. ........ r60134 | georg.brandl | 2008-01-20 13:05:43 +0100 (Sun, 20 Jan 2008) | 4 lines #856047: respect the ``no_proxy`` env var when checking for proxies in urllib and using the other ``_proxy`` env vars. Original patch by Donovan Baarda. ........ r60135 | georg.brandl | 2008-01-20 13:18:17 +0100 (Sun, 20 Jan 2008) | 4 lines #1664522: in urllib, don't read non-existing directories in ftp mode, returning a 0-byte file -- raise an IOError instead. Original patch from Phil Knirsch. ........ r60136 | georg.brandl | 2008-01-20 13:57:47 +0100 (Sun, 20 Jan 2008) | 2 lines #799369: document possible sys.platform values. ........ r60137 | georg.brandl | 2008-01-20 14:08:37 +0100 (Sun, 20 Jan 2008) | 2 lines #652749: document the constants added to the builtins by site.py. ........ r60138 | georg.brandl | 2008-01-20 14:59:46 +0100 (Sun, 20 Jan 2008) | 2 lines #1648: add sys.gettrace() and sys.getprofile(). ........ r60139 | georg.brandl | 2008-01-20 15:17:42 +0100 (Sun, 20 Jan 2008) | 2 lines #1669: don't allow shutil.rmtree() to be called on a symlink. ........ r60140 | georg.brandl | 2008-01-20 15:20:02 +0100 (Sun, 20 Jan 2008) | 2 lines Fix test_pyclbr after urllib change. ........ r60141 | christian.heimes | 2008-01-20 15:28:28 +0100 (Sun, 20 Jan 2008) | 1 line Fixed a wrong assumption in configure.in and Include/pyport.h. The is finite function is not called isfinite() but finite(). Sorry, my fault. :) ........ r60142 | georg.brandl | 2008-01-20 15:31:27 +0100 (Sun, 20 Jan 2008) | 2 lines #1876: fix typos in test_operator. ........
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.. note::
The names :data:`None`, :data:`False`, :data:`True` and :data:`__debug__`
cannot be reassigned (assignments to them, even as an attribute name, raise
:exc:`SyntaxError`), so they can be considered "true" constants.
Merged revisions 60124-60142 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r60131 | georg.brandl | 2008-01-20 12:13:29 +0100 (Sun, 20 Jan 2008) | 3 lines #1351692: in pprint, always call format() for dict and list items to enable custom formatting of contents via subclassing PrettyPrinter. ........ r60133 | georg.brandl | 2008-01-20 12:43:03 +0100 (Sun, 20 Jan 2008) | 2 lines #1178141: add addinfourl.code to get http status code from urllib. ........ r60134 | georg.brandl | 2008-01-20 13:05:43 +0100 (Sun, 20 Jan 2008) | 4 lines #856047: respect the ``no_proxy`` env var when checking for proxies in urllib and using the other ``_proxy`` env vars. Original patch by Donovan Baarda. ........ r60135 | georg.brandl | 2008-01-20 13:18:17 +0100 (Sun, 20 Jan 2008) | 4 lines #1664522: in urllib, don't read non-existing directories in ftp mode, returning a 0-byte file -- raise an IOError instead. Original patch from Phil Knirsch. ........ r60136 | georg.brandl | 2008-01-20 13:57:47 +0100 (Sun, 20 Jan 2008) | 2 lines #799369: document possible sys.platform values. ........ r60137 | georg.brandl | 2008-01-20 14:08:37 +0100 (Sun, 20 Jan 2008) | 2 lines #652749: document the constants added to the builtins by site.py. ........ r60138 | georg.brandl | 2008-01-20 14:59:46 +0100 (Sun, 20 Jan 2008) | 2 lines #1648: add sys.gettrace() and sys.getprofile(). ........ r60139 | georg.brandl | 2008-01-20 15:17:42 +0100 (Sun, 20 Jan 2008) | 2 lines #1669: don't allow shutil.rmtree() to be called on a symlink. ........ r60140 | georg.brandl | 2008-01-20 15:20:02 +0100 (Sun, 20 Jan 2008) | 2 lines Fix test_pyclbr after urllib change. ........ r60141 | christian.heimes | 2008-01-20 15:28:28 +0100 (Sun, 20 Jan 2008) | 1 line Fixed a wrong assumption in configure.in and Include/pyport.h. The is finite function is not called isfinite() but finite(). Sorry, my fault. :) ........ r60142 | georg.brandl | 2008-01-20 15:31:27 +0100 (Sun, 20 Jan 2008) | 2 lines #1876: fix typos in test_operator. ........
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Constants added by the :mod:`site` module
-----------------------------------------
The :mod:`site` module (which is imported automatically during startup, except
if the :option:`-S` command-line option is given) adds several constants to the
built-in namespace. They are useful for the interactive interpreter shell and
should not be used in programs.
.. data:: quit(code=None)
exit(code=None)
Merged revisions 60124-60142 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r60131 | georg.brandl | 2008-01-20 12:13:29 +0100 (Sun, 20 Jan 2008) | 3 lines #1351692: in pprint, always call format() for dict and list items to enable custom formatting of contents via subclassing PrettyPrinter. ........ r60133 | georg.brandl | 2008-01-20 12:43:03 +0100 (Sun, 20 Jan 2008) | 2 lines #1178141: add addinfourl.code to get http status code from urllib. ........ r60134 | georg.brandl | 2008-01-20 13:05:43 +0100 (Sun, 20 Jan 2008) | 4 lines #856047: respect the ``no_proxy`` env var when checking for proxies in urllib and using the other ``_proxy`` env vars. Original patch by Donovan Baarda. ........ r60135 | georg.brandl | 2008-01-20 13:18:17 +0100 (Sun, 20 Jan 2008) | 4 lines #1664522: in urllib, don't read non-existing directories in ftp mode, returning a 0-byte file -- raise an IOError instead. Original patch from Phil Knirsch. ........ r60136 | georg.brandl | 2008-01-20 13:57:47 +0100 (Sun, 20 Jan 2008) | 2 lines #799369: document possible sys.platform values. ........ r60137 | georg.brandl | 2008-01-20 14:08:37 +0100 (Sun, 20 Jan 2008) | 2 lines #652749: document the constants added to the builtins by site.py. ........ r60138 | georg.brandl | 2008-01-20 14:59:46 +0100 (Sun, 20 Jan 2008) | 2 lines #1648: add sys.gettrace() and sys.getprofile(). ........ r60139 | georg.brandl | 2008-01-20 15:17:42 +0100 (Sun, 20 Jan 2008) | 2 lines #1669: don't allow shutil.rmtree() to be called on a symlink. ........ r60140 | georg.brandl | 2008-01-20 15:20:02 +0100 (Sun, 20 Jan 2008) | 2 lines Fix test_pyclbr after urllib change. ........ r60141 | christian.heimes | 2008-01-20 15:28:28 +0100 (Sun, 20 Jan 2008) | 1 line Fixed a wrong assumption in configure.in and Include/pyport.h. The is finite function is not called isfinite() but finite(). Sorry, my fault. :) ........ r60142 | georg.brandl | 2008-01-20 15:31:27 +0100 (Sun, 20 Jan 2008) | 2 lines #1876: fix typos in test_operator. ........
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Objects that when printed, print a message like "Use quit() or Ctrl-D
(i.e. EOF) to exit", and when called, raise :exc:`SystemExit` with the
Merged revisions 74277,74321,74323,74326,74355,74465,74467,74488,74492,74513,74531,74549,74553,74625,74632,74643-74644,74647,74652,74666,74671,74727,74739 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r74277 | sean.reifschneider | 2009-08-01 18:54:55 -0500 (Sat, 01 Aug 2009) | 3 lines - Issue #6624: yArg_ParseTuple with "s" format when parsing argument with NUL: Bogus TypeError detail string. ........ r74321 | guilherme.polo | 2009-08-05 11:51:41 -0500 (Wed, 05 Aug 2009) | 1 line Easier reference to find (at least while svn continues being used). ........ r74323 | guilherme.polo | 2009-08-05 18:48:26 -0500 (Wed, 05 Aug 2009) | 1 line Typo. ........ r74326 | jesse.noller | 2009-08-05 21:05:56 -0500 (Wed, 05 Aug 2009) | 1 line Fix issue 4660: spurious task_done errors in multiprocessing, remove doc note for from_address ........ r74355 | gregory.p.smith | 2009-08-12 12:02:37 -0500 (Wed, 12 Aug 2009) | 2 lines comment typo fix ........ r74465 | vinay.sajip | 2009-08-15 18:23:12 -0500 (Sat, 15 Aug 2009) | 1 line Added section on logging to one file from multiple processes. ........ r74467 | vinay.sajip | 2009-08-15 18:34:47 -0500 (Sat, 15 Aug 2009) | 1 line Refined section on logging to one file from multiple processes. ........ r74488 | vinay.sajip | 2009-08-17 08:14:37 -0500 (Mon, 17 Aug 2009) | 1 line Further refined section on logging to one file from multiple processes. ........ r74492 | r.david.murray | 2009-08-17 14:26:49 -0500 (Mon, 17 Aug 2009) | 2 lines Issue 6685: 'toupper' -> 'upper' in cgi doc example explanation. ........ r74513 | skip.montanaro | 2009-08-18 09:37:52 -0500 (Tue, 18 Aug 2009) | 1 line missing module ref (issue6723) ........ r74531 | vinay.sajip | 2009-08-20 17:04:32 -0500 (Thu, 20 Aug 2009) | 1 line Added section on exceptions raised during logging. ........ r74549 | benjamin.peterson | 2009-08-24 12:42:36 -0500 (Mon, 24 Aug 2009) | 1 line fix pdf building by teaching latex the right encoding package ........ r74553 | r.david.murray | 2009-08-26 20:04:59 -0500 (Wed, 26 Aug 2009) | 2 lines Remove leftover text from end of sentence. ........ r74625 | benjamin.peterson | 2009-09-01 17:27:57 -0500 (Tue, 01 Sep 2009) | 1 line remove the check that classmethod's argument is a callable ........ r74632 | georg.brandl | 2009-09-03 02:27:26 -0500 (Thu, 03 Sep 2009) | 1 line #6828: fix wrongly highlighted blocks. ........ r74643 | georg.brandl | 2009-09-04 01:59:20 -0500 (Fri, 04 Sep 2009) | 2 lines Issue #2666: Handle BROWSER environment variable properly for unknown browser names in the webbrowser module. ........ r74644 | georg.brandl | 2009-09-04 02:55:14 -0500 (Fri, 04 Sep 2009) | 1 line #5047: remove Monterey support from configure. ........ r74647 | georg.brandl | 2009-09-04 03:17:04 -0500 (Fri, 04 Sep 2009) | 2 lines Issue #5275: In Cookie's Cookie.load(), properly handle non-string arguments as documented. ........ r74652 | georg.brandl | 2009-09-04 06:25:37 -0500 (Fri, 04 Sep 2009) | 1 line #6756: add some info about the "acct" parameter. ........ r74666 | georg.brandl | 2009-09-05 04:04:09 -0500 (Sat, 05 Sep 2009) | 1 line #6841: remove duplicated word. ........ r74671 | georg.brandl | 2009-09-05 11:47:17 -0500 (Sat, 05 Sep 2009) | 1 line #6843: add link from filterwarnings to where the meaning of the arguments is covered. ........ r74727 | benjamin.peterson | 2009-09-08 18:04:22 -0500 (Tue, 08 Sep 2009) | 1 line #6865 fix ref counting in initialization of pwd module ........ r74739 | georg.brandl | 2009-09-11 02:55:20 -0500 (Fri, 11 Sep 2009) | 1 line Move function back to its section. ........
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specified exit code.
Merged revisions 60124-60142 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r60131 | georg.brandl | 2008-01-20 12:13:29 +0100 (Sun, 20 Jan 2008) | 3 lines #1351692: in pprint, always call format() for dict and list items to enable custom formatting of contents via subclassing PrettyPrinter. ........ r60133 | georg.brandl | 2008-01-20 12:43:03 +0100 (Sun, 20 Jan 2008) | 2 lines #1178141: add addinfourl.code to get http status code from urllib. ........ r60134 | georg.brandl | 2008-01-20 13:05:43 +0100 (Sun, 20 Jan 2008) | 4 lines #856047: respect the ``no_proxy`` env var when checking for proxies in urllib and using the other ``_proxy`` env vars. Original patch by Donovan Baarda. ........ r60135 | georg.brandl | 2008-01-20 13:18:17 +0100 (Sun, 20 Jan 2008) | 4 lines #1664522: in urllib, don't read non-existing directories in ftp mode, returning a 0-byte file -- raise an IOError instead. Original patch from Phil Knirsch. ........ r60136 | georg.brandl | 2008-01-20 13:57:47 +0100 (Sun, 20 Jan 2008) | 2 lines #799369: document possible sys.platform values. ........ r60137 | georg.brandl | 2008-01-20 14:08:37 +0100 (Sun, 20 Jan 2008) | 2 lines #652749: document the constants added to the builtins by site.py. ........ r60138 | georg.brandl | 2008-01-20 14:59:46 +0100 (Sun, 20 Jan 2008) | 2 lines #1648: add sys.gettrace() and sys.getprofile(). ........ r60139 | georg.brandl | 2008-01-20 15:17:42 +0100 (Sun, 20 Jan 2008) | 2 lines #1669: don't allow shutil.rmtree() to be called on a symlink. ........ r60140 | georg.brandl | 2008-01-20 15:20:02 +0100 (Sun, 20 Jan 2008) | 2 lines Fix test_pyclbr after urllib change. ........ r60141 | christian.heimes | 2008-01-20 15:28:28 +0100 (Sun, 20 Jan 2008) | 1 line Fixed a wrong assumption in configure.in and Include/pyport.h. The is finite function is not called isfinite() but finite(). Sorry, my fault. :) ........ r60142 | georg.brandl | 2008-01-20 15:31:27 +0100 (Sun, 20 Jan 2008) | 2 lines #1876: fix typos in test_operator. ........
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.. data:: copyright
license
credits
Objects that when printed, print a message like "Type license() to see the
full license text", and when called, display the corresponding text in a
pager-like fashion (one screen at a time).