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      ``NotImplementedError`` 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.. index:: single: ...; ellipsis literal
55.. data:: Ellipsis
56
57   The same as the ellipsis literal "``...``".  Special value used mostly in conjunction
58   with extended slicing syntax for user-defined container data types.
59
60
61.. data:: __debug__
62
63   This constant is true if Python was not started with an :option:`-O` option.
64   See also the :keyword:`assert` statement.
65
66
67.. note::
68
69   The names :data:`None`, :data:`False`, :data:`True` and :data:`__debug__`
70   cannot be reassigned (assignments to them, even as an attribute name, raise
71   :exc:`SyntaxError`), so they can be considered "true" constants.
72
73
74Constants added by the :mod:`site` module
75-----------------------------------------
76
77The :mod:`site` module (which is imported automatically during startup, except
78if the :option:`-S` command-line option is given) adds several constants to the
79built-in namespace.  They are useful for the interactive interpreter shell and
80should not be used in programs.
81
82.. data:: quit(code=None)
83          exit(code=None)
84
85   Objects that when printed, print a message like "Use quit() or Ctrl-D
86   (i.e. EOF) to exit", and when called, raise :exc:`SystemExit` with the
87   specified exit code.
88
89.. data:: copyright
90          credits
91
92   Objects that when printed or called, print the text of copyright or
93   credits, respectively.
94
95.. data:: license
96
97   Object that when printed, prints the message "Type license() to see the
98   full license text", and when called, displays the full license text in a
99   pager-like fashion (one screen at a time).
100