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/Doc/library/constants.rst

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 1Built-in Constants
 2==================
 3
 4A small number of constants live in the built-in namespace.  They are:
 5
 6
 7.. data:: False
 8
 9   The false value of the :class:`bool` type.
10
11   .. versionadded:: 2.3
12
13
14.. data:: True
15
16   The true value of the :class:`bool` type.
17
18   .. versionadded:: 2.3
19
20
21.. data:: None
22
23   The sole value of :attr:`types.NoneType`.  ``None`` is frequently used to
24   represent the absence of a value, as when default arguments are not passed to a
25   function.
26
27   .. versionchanged:: 2.4
28      Assignments to ``None`` are illegal and raise a :exc:`SyntaxError`.
29
30
31.. data:: NotImplemented
32
33   Special value which can be returned by the "rich comparison" special methods
34   (:meth:`__eq__`, :meth:`__lt__`, and friends), to indicate that the comparison
35   is not implemented with respect to the other type.
36
37
38.. data:: Ellipsis
39
40   Special value used in conjunction with extended slicing syntax.
41
42   .. XXX Someone who understands extended slicing should fill in here.
43
44
45.. data:: __debug__
46
47   This constant is true if Python was not started with an :option:`-O` option.
48   Assignments to :const:`__debug__` are illegal and raise a :exc:`SyntaxError`.
49   See also the :keyword:`assert` statement.
50
51
52Constants added by the :mod:`site` module
53-----------------------------------------
54
55The :mod:`site` module (which is imported automatically during startup, except
56if the :option:`-S` command-line option is given) adds several constants to the
57built-in namespace.  They are useful for the interactive interpreter shell and
58should not be used in programs.
59
60.. data:: quit([code=None])
61          exit([code=None])
62
63   Objects that when printed, print a message like "Use quit() or Ctrl-D
64   (i.e. EOF) to exit", and when called, raise :exc:`SystemExit` with the
65   specified exit code, and when .
66
67.. data:: copyright
68          license
69          credits
70
71   Objects that when printed, print a message like "Type license() to see the
72   full license text", and when called, display the corresponding text in a
73   pager-like fashion (one screen at a time).