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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, so they can be considered "true" constants.
.. XXX False, True, None are keywords too
.. data:: False
The false value of the :class:`bool` type.
.. data:: True
The true value of the :class:`bool` type.
.. 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.
.. 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 ::
val = container[1:5, 7:10, ...]
.. data:: __debug__
A boolean value that is :data:`True` if Python was not started with the
``-O`` command line option. Its value is used indirectly by the
:keyword:`assert` statement, but it can also be used directly in code.
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