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104 lines
3.6 KiB
ReStructuredText
104 lines
3.6 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 the type ``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 should be returned by the binary special methods
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(e.g. :meth:`__eq__`, :meth:`__lt__`, :meth:`__add__`, :meth:`__rsub__`,
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etc.) to indicate that the operation is not implemented with respect to
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the other type; may be returned by the in-place binary special methods
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(e.g. :meth:`__imul__`, :meth:`__iand__`, etc.) for the same purpose.
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It should not be evaluated in a boolean context.
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.. note::
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When a binary (or in-place) method returns ``NotImplemented`` the
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interpreter will try the reflected operation on the other type (or some
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other fallback, depending on the operator). If all attempts return
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``NotImplemented``, the interpreter will raise an appropriate exception.
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Incorrectly returning ``NotImplemented`` will result in a misleading
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error message or the ``NotImplemented`` value being returned to Python code.
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See :ref:`implementing-the-arithmetic-operations` for examples.
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.. note::
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``NotImplementedError`` and ``NotImplemented`` are not interchangeable,
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even though they have similar names and purposes.
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See :exc:`NotImplementedError` for details on when to use it.
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.. versionchanged:: 3.9
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Evaluating ``NotImplemented`` in a boolean context is deprecated. While
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it currently evaluates as true, it will emit a :exc:`DeprecationWarning`.
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It will raise a :exc:`TypeError` in a future version of Python.
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.. index:: single: ...; ellipsis literal
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.. data:: Ellipsis
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The same as the ellipsis literal "``...``". Special value used mostly in conjunction
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with extended 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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credits
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Objects that when printed or called, print the text of copyright or
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credits, respectively.
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.. data:: license
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Object that when printed, prints the message "Type license() to see the
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full license text", and when called, displays the full license text in a
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pager-like fashion (one screen at a time).
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