cpython/Doc/library/constants.rst

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Built-in Constants
==================
A small number of constants live in the built-in namespace. They are:
.. data:: False
The false value of the :class:`bool` type.
.. versionadded:: 2.3
.. data:: True
The true value of the :class:`bool` type.
.. versionadded:: 2.3
.. data:: None
The sole value of :attr:`types.NoneType`. ``None`` is frequently used to
represent the absence of a value, as when default arguments are not passed to a
function.
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.. versionchanged:: 2.4
Assignments to ``None`` are illegal and raise a :exc:`SyntaxError`.
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.. data:: NotImplemented
Special value which can be returned by the "rich comparison" special methods
(:meth:`__eq__`, :meth:`__lt__`, and friends), to indicate that the comparison
is not implemented with respect to the other type.
.. data:: Ellipsis
Special value used in conjunction with extended slicing syntax.
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.. XXX Someone who understands extended slicing should fill in here.
.. data:: __debug__
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This constant is true if Python was not started with an :option:`-O` option.
Assignments to :const:`__debug__` are illegal and raise a :exc:`SyntaxError`.
See also the :keyword:`assert` statement.