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Éric Araujo96deb752011-06-08 04:53:20 +02001.. _built-in-consts:
2
Georg Brandl116aa622007-08-15 14:28:22 +00003Built-in Constants
4==================
5
6A small number of constants live in the built-in namespace. They are:
7
Georg Brandl116aa622007-08-15 14:28:22 +00008.. data:: False
9
Georg Brandl48310cd2009-01-03 21:18:54 +000010 The false value of the :class:`bool` type. Assignments to ``False``
Christian Heimes5b5e81c2007-12-31 16:14:33 +000011 are illegal and raise a :exc:`SyntaxError`.
Georg Brandl116aa622007-08-15 14:28:22 +000012
Georg Brandl116aa622007-08-15 14:28:22 +000013
14.. data:: True
15
Georg Brandl48310cd2009-01-03 21:18:54 +000016 The true value of the :class:`bool` type. Assignments to ``True``
Christian Heimes5b5e81c2007-12-31 16:14:33 +000017 are illegal and raise a :exc:`SyntaxError`.
Georg Brandl116aa622007-08-15 14:28:22 +000018
Georg Brandl116aa622007-08-15 14:28:22 +000019
20.. data:: None
21
22 The sole value of :attr:`types.NoneType`. ``None`` is frequently used to
23 represent the absence of a value, as when default arguments are not passed to a
Christian Heimes5b5e81c2007-12-31 16:14:33 +000024 function. Assignments to ``None`` are illegal and raise a :exc:`SyntaxError`.
Georg Brandl116aa622007-08-15 14:28:22 +000025
26
27.. data:: NotImplemented
28
29 Special value which can be returned by the "rich comparison" special methods
30 (:meth:`__eq__`, :meth:`__lt__`, and friends), to indicate that the comparison
31 is not implemented with respect to the other type.
32
33
34.. data:: Ellipsis
35
Georg Brandl9499bb72010-08-02 19:35:06 +000036 The same as ``...``. Special value used mostly in conjunction with extended
37 slicing syntax for user-defined container data types.
Georg Brandl96593ed2007-09-07 14:15:41 +000038
39
40.. data:: __debug__
41
Christian Heimes5b5e81c2007-12-31 16:14:33 +000042 This constant is true if Python was not started with an :option:`-O` option.
Christian Heimes5b5e81c2007-12-31 16:14:33 +000043 See also the :keyword:`assert` statement.
Christian Heimes9bd667a2008-01-20 15:14:11 +000044
Georg Brandl0c7ade22010-08-02 19:39:17 +000045
Georg Brandlc589a702010-08-02 19:36:36 +000046.. note::
47
48 The names :data:`None`, :data:`False`, :data:`True` and :data:`__debug__`
49 cannot be reassigned (assignments to them, even as an attribute name, raise
50 :exc:`SyntaxError`), so they can be considered "true" constants.
51
Christian Heimes9bd667a2008-01-20 15:14:11 +000052
53Constants added by the :mod:`site` module
54-----------------------------------------
55
56The :mod:`site` module (which is imported automatically during startup, except
57if the :option:`-S` command-line option is given) adds several constants to the
58built-in namespace. They are useful for the interactive interpreter shell and
59should not be used in programs.
60
Georg Brandlc2a4f4f2009-04-10 09:03:43 +000061.. data:: quit(code=None)
62 exit(code=None)
Christian Heimes9bd667a2008-01-20 15:14:11 +000063
64 Objects that when printed, print a message like "Use quit() or Ctrl-D
65 (i.e. EOF) to exit", and when called, raise :exc:`SystemExit` with the
Benjamin Peterson8719ad52009-09-11 22:24:02 +000066 specified exit code.
Christian Heimes9bd667a2008-01-20 15:14:11 +000067
68.. data:: copyright
69 license
70 credits
71
72 Objects that when printed, print a message like "Type license() to see the
73 full license text", and when called, display the corresponding text in a
74 pager-like fashion (one screen at a time).
75