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1.. _built-in-consts:
2
3Built-in Constants
4==================
5
6A small number of constants live in the built-in namespace.  They are:
7
8.. data:: False
9
10   The false value of the :class:`bool` type. Assignments to ``False``
11   are illegal and raise a :exc:`SyntaxError`.
12
13
14.. data:: True
15
16   The true value of the :class:`bool` type. Assignments to ``True``
17   are illegal and raise a :exc:`SyntaxError`.
18
19
20.. data:: None
21
22   The sole value of the type ``NoneType``.  ``None`` is frequently used to
23   represent the absence of a value, as when default arguments are not passed to a
24   function. Assignments to ``None`` are illegal and raise a :exc:`SyntaxError`.
25
26
27.. data:: NotImplemented
28
29   Special value which should be returned by the binary special methods
30   (e.g. :meth:`__eq__`, :meth:`__lt__`, :meth:`__add__`, :meth:`__rsub__`,
31   etc.) to indicate that the operation is not implemented with respect to
32   the other type; may be returned by the in-place binary special methods
33   (e.g. :meth:`__imul__`, :meth:`__iand__`, etc.) for the same purpose.
34   Its truth value is true.
35
36   .. note::
37
38      When a binary (or in-place) method returns ``NotImplemented`` the
39      interpreter will try the reflected operation on the other type (or some
40      other fallback, depending on the operator).  If all attempts return
41      ``NotImplemented``, the interpreter will raise an appropriate exception.
42      Incorrectly returning ``NotImplemented`` will result in a misleading
43      error message or the ``NotImplemented`` value being returned to Python code.
44
45      See :ref:`implementing-the-arithmetic-operations` for examples.
46
47   .. note::
48
49      ``NotImplentedError`` and ``NotImplemented`` are not interchangeable,
50      even though they have similar names and purposes.
51      See :exc:`NotImplementedError` for details on when to use it.
52
53
54.. data:: Ellipsis
55
56   The same as ``...``.  Special value used mostly in conjunction with extended
57   slicing syntax for user-defined container data types.
58
59
60.. data:: __debug__
61
62   This constant is true if Python was not started with an :option:`-O` option.
63   See also the :keyword:`assert` statement.
64
65
66.. note::
67
68   The names :data:`None`, :data:`False`, :data:`True` and :data:`__debug__`
69   cannot be reassigned (assignments to them, even as an attribute name, raise
70   :exc:`SyntaxError`), so they can be considered "true" constants.
71
72
73Constants added by the :mod:`site` module
74-----------------------------------------
75
76The :mod:`site` module (which is imported automatically during startup, except
77if the :option:`-S` command-line option is given) adds several constants to the
78built-in namespace.  They are useful for the interactive interpreter shell and
79should not be used in programs.
80
81.. data:: quit(code=None)
82          exit(code=None)
83
84   Objects that when printed, print a message like "Use quit() or Ctrl-D
85   (i.e. EOF) to exit", and when called, raise :exc:`SystemExit` with the
86   specified exit code.
87
88.. data:: copyright
89          license
90          credits
91
92   Objects that when printed, print a message like "Type license() to see the
93   full license text", and when called, display the corresponding text in a
94   pager-like fashion (one screen at a time).
95
96