1 .. _built-in-consts: 2 3 Built-in Constants 4 ================== 5 6 A 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. 11 12 .. versionadded:: 2.3 13 14 15 .. data:: True 16 17 The true value of the :class:`bool` type. 18 19 .. versionadded:: 2.3 20 21 22 .. data:: None 23 24 The sole value of :attr:`types.NoneType`. ``None`` is frequently used to 25 represent the absence of a value, as when default arguments are not passed to a 26 function. 27 28 .. versionchanged:: 2.4 29 Assignments to ``None`` are illegal and raise a :exc:`SyntaxError`. 30 31 32 .. data:: NotImplemented 33 34 Special value which can be returned by the "rich comparison" special methods 35 (:meth:`__eq__`, :meth:`__lt__`, and friends), to indicate that the comparison 36 is not implemented with respect to the other type. 37 38 39 .. data:: Ellipsis 40 41 Special value used in conjunction with extended slicing syntax. 42 43 44 .. data:: __debug__ 45 46 This constant is true if Python was not started with an :option:`-O` option. 47 See also the :keyword:`assert` statement. 48 49 50 .. note:: 51 52 The names :data:`None` and :data:`__debug__` cannot be reassigned 53 (assignments to them, even as an attribute name, raise :exc:`SyntaxError`), 54 so they can be considered "true" constants. 55 56 .. versionchanged:: 2.7 57 Assignments to ``__debug__`` as an attribute became illegal. 58 59 60 Constants added by the :mod:`site` module 61 ----------------------------------------- 62 63 The :mod:`site` module (which is imported automatically during startup, except 64 if the :option:`-S` command-line option is given) adds several constants to the 65 built-in namespace. They are useful for the interactive interpreter shell and 66 should not be used in programs. 67 68 .. data:: quit([code=None]) 69 exit([code=None]) 70 71 Objects that when printed, print a message like "Use quit() or Ctrl-D 72 (i.e. EOF) to exit", and when called, raise :exc:`SystemExit` with the 73 specified exit code. 74 75 .. data:: copyright 76 license 77 credits 78 79 Objects that when printed, print a message like "Type license() to see the 80 full license text", and when called, display the corresponding text in a 81 pager-like fashion (one screen at a time). 82
