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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. 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.
.. 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).
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