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