.. _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. 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 ``NotImplemented`` is returned, the interpreter will then try the reflected operation on the other type, or some other fallback, depending on the operator. If all attempted operations return ``NotImplemented``, the interpreter will raise an appropriate exception. See :ref:`implementing-the-arithmetic-operations` for more details. .. data:: Ellipsis The same as ``...``. 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. 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).