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.

   .. XXX Someone who understands extended slicing should fill in here.

.. data:: __debug__

   This constant is true if Python was not started with an :option:`-O` option.

   Assignments to :const:`__debug__` are illegal and raise a :exc:`SyntaxError`.

   See also the :keyword:`assert` statement.

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, and when .

.. 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).