1 Built-in Constants

     2 ==================

     3 

     4 A 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 

    52 Constants added by the :mod:`site` module

    53 -----------------------------------------

    54 

    55 The :mod:`site` module (which is imported automatically during startup, except

    56 if the :option:`-S` command-line option is given) adds several constants to the

    57 built-in namespace.  They are useful for the interactive interpreter shell and

    58 should 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).