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Built-in Constants
==================
A small number of constants live in the built-in namespace. They are:
.. note::
:data:`None`, :data:`False`, :data:`True` and :data:`__debug__` cannot be
reassigned (assignments to them raise :exc:`SyntaxError`), so they can be
considered "true" constants.
.. XXX False, True, None are keywords too
.. data:: False
The false value of the :class:`bool` type. Assignments to ``False``
are illegal and raise a :exc:`SyntaxError`.
.. data:: True
The true value of the :class:`bool` type. Assignments to ``True``
are illegal and raise a :exc:`SyntaxError`.
.. 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. Assignments to ``None`` are illegal and raise a :exc:`SyntaxError`.
.. 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
The same as ``...``. Special value used mostly in conjunction with extended
slicing syntax for user-defined container data types, as in ::
.. XXX Someone who understands extended slicing should fill in here.
.. data:: __debug__
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.
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