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75 lines
2.2 KiB
ReStructuredText
75 lines
2.2 KiB
ReStructuredText
.. _built-in-consts:
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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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.. 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.
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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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See also the :keyword:`assert` statement.
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.. note::
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The names :data:`None`, :data:`False`, :data:`True` and :data:`__debug__`
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cannot be reassigned (assignments to them, even as an attribute name, raise
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:exc:`SyntaxError`), so they can be considered "true" constants.
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Constants added by the :mod:`site` module
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-----------------------------------------
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The :mod:`site` module (which is imported automatically during startup, except
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if the :option:`-S` command-line option is given) adds several constants to the
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built-in namespace. They are useful for the interactive interpreter shell and
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should not be used in programs.
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.. data:: quit(code=None)
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exit(code=None)
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Objects that when printed, print a message like "Use quit() or Ctrl-D
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(i.e. EOF) to exit", and when called, raise :exc:`SystemExit` with the
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specified exit code.
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.. data:: copyright
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license
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credits
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Objects that when printed, print a message like "Type license() to see the
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full license text", and when called, display the corresponding text in a
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pager-like fashion (one screen at a time).
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