A small number of constants live in the built-in namespace. They are:
.. data:: False The false value of the :class:`bool` type. Assignments to ``False`` are illegal and raise a :exc:`SyntaxError`.
.. data:: True The true value of the :class:`bool` type. Assignments to ``True`` are illegal and raise a :exc:`SyntaxError`.
.. data:: None The sole value of the type ``NoneType``. ``None`` is frequently used to represent the absence of a value, as when default arguments are not passed to a function. Assignments to ``None`` are illegal and raise a :exc:`SyntaxError`.
.. data:: NotImplemented Special value which should be returned by the binary special methods (e.g. :meth:`__eq__`, :meth:`__lt__`, :meth:`__add__`, :meth:`__rsub__`, etc.) to indicate that the operation is not implemented with respect to the other type; may be returned by the in-place binary special methods (e.g. :meth:`__imul__`, :meth:`__iand__`, etc.) for the same purpose. Its truth value is true. .. note:: When a binary (or in-place) method returns ``NotImplemented`` the interpreter will try the reflected operation on the other type (or some other fallback, depending on the operator). If all attempts return ``NotImplemented``, the interpreter will raise an appropriate exception. Incorrectly returning ``NotImplemented`` will result in a misleading error message or the ``NotImplemented`` value being returned to Python code. See :ref:`implementing-the-arithmetic-operations` for examples. .. note:: ``NotImplementedError`` and ``NotImplemented`` are not interchangeable, even though they have similar names and purposes. See :exc:`NotImplementedError` for details on when to use it.
.. index:: single: ...; ellipsis literal
.. data:: Ellipsis The same as the ellipsis literal "``...``". Special value used mostly in conjunction with extended slicing syntax for user-defined container data types.
.. 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`, :data:`False`, :data:`True` and :data:`__debug__` cannot be reassigned (assignments to them, even as an attribute name, raise :exc:`SyntaxError`), so they can be considered "true" constants.
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 credits Objects that when printed or called, print the text of copyright or credits, respectively.
.. data:: license Object that when printed, prints the message "Type license() to see the full license text", and when called, displays the full license text in a pager-like fashion (one screen at a time).
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