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.. _built-in-consts: Built-in Constants ================== A small number of constants live in the built-in namespace. They are: .. data:: False The false value of the :class:`bool` type. .. versionadded:: 2.3 .. data:: True The true value of the :class:`bool` type. .. versionadded:: 2.3 .. 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. .. versionchanged:: 2.4 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 Special value used in conjunction with extended slicing syntax. .. data:: __debug__ This constant is true if Python was not started with an :option:`-O` option. See also the :keyword:`assert` statement. .. note:: The names :data:`None` and :data:`__debug__` cannot be reassigned (assignments to them, even as an attribute name, raise :exc:`SyntaxError`), so they can be considered "true" constants. .. versionchanged:: 2.7 Assignments to ``__debug__`` as an attribute became illegal. Constants added by the :mod:`site` module ----------------------------------------- The :mod:`site` module (which is imported automatically during startup, except if the :option:`-S` command-line option is given) adds several constants to the built-in namespace. They are useful for the interactive interpreter shell and should not be used in programs. .. data:: quit([code=None]) exit([code=None]) Objects that when printed, print a message like "Use quit() or Ctrl-D (i.e. EOF) to exit", and when called, raise :exc:`SystemExit` with the specified exit code. .. data:: copyright license credits Objects that when printed, print a message like "Type license() to see the full license text", and when called, display the corresponding text in a pager-like fashion (one screen at a time).