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 An object frequently used to represent the absence of a value, as when 23 default arguments are not passed to a function. Assignments to ``None`` 24 are illegal and raise a :exc:`SyntaxError`. 25 ``None`` is the sole instance of the :data:`NoneType` type. 26 27 28.. data:: NotImplemented 29 30 A special value which should be returned by the binary special methods 31 (e.g. :meth:`__eq__`, :meth:`__lt__`, :meth:`__add__`, :meth:`__rsub__`, 32 etc.) to indicate that the operation is not implemented with respect to 33 the other type; may be returned by the in-place binary special methods 34 (e.g. :meth:`__imul__`, :meth:`__iand__`, etc.) for the same purpose. 35 It should not be evaluated in a boolean context. 36 ``NotImplemented`` is the sole instance of the :data:`types.NotImplementedType` type. 37 38 .. note:: 39 40 When a binary (or in-place) method returns ``NotImplemented`` the 41 interpreter will try the reflected operation on the other type (or some 42 other fallback, depending on the operator). If all attempts return 43 ``NotImplemented``, the interpreter will raise an appropriate exception. 44 Incorrectly returning ``NotImplemented`` will result in a misleading 45 error message or the ``NotImplemented`` value being returned to Python code. 46 47 See :ref:`implementing-the-arithmetic-operations` for examples. 48 49 .. note:: 50 51 ``NotImplementedError`` and ``NotImplemented`` are not interchangeable, 52 even though they have similar names and purposes. 53 See :exc:`NotImplementedError` for details on when to use it. 54 55 .. versionchanged:: 3.9 56 Evaluating ``NotImplemented`` in a boolean context is deprecated. While 57 it currently evaluates as true, it will emit a :exc:`DeprecationWarning`. 58 It will raise a :exc:`TypeError` in a future version of Python. 59 60 61.. index:: single: ...; ellipsis literal 62.. data:: Ellipsis 63 64 The same as the ellipsis literal "``...``". Special value used mostly in conjunction 65 with extended slicing syntax for user-defined container data types. 66 ``Ellipsis`` is the sole instance of the :data:`types.EllipsisType` type. 67 68 69.. data:: __debug__ 70 71 This constant is true if Python was not started with an :option:`-O` option. 72 See also the :keyword:`assert` statement. 73 74 75.. note:: 76 77 The names :data:`None`, :data:`False`, :data:`True` and :data:`__debug__` 78 cannot be reassigned (assignments to them, even as an attribute name, raise 79 :exc:`SyntaxError`), so they can be considered "true" constants. 80 81 82Constants added by the :mod:`site` module 83----------------------------------------- 84 85The :mod:`site` module (which is imported automatically during startup, except 86if the :option:`-S` command-line option is given) adds several constants to the 87built-in namespace. They are useful for the interactive interpreter shell and 88should not be used in programs. 89 90.. data:: quit(code=None) 91 exit(code=None) 92 93 Objects that when printed, print a message like "Use quit() or Ctrl-D 94 (i.e. EOF) to exit", and when called, raise :exc:`SystemExit` with the 95 specified exit code. 96 97.. data:: copyright 98 credits 99 100 Objects that when printed or called, print the text of copyright or 101 credits, respectively. 102 103.. data:: license 104 105 Object that when printed, prints the message "Type license() to see the 106 full license text", and when called, displays the full license text in a 107 pager-like fashion (one screen at a time). 108