.. _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

An object 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. None is the sole instance of the :data:~types.NoneType type.

.. data:: NotImplemented

A special value which should be returned by the binary special methods (e.g. :meth:~object.__eq__, :meth:~object.__lt__, :meth:~object.__add__, :meth:~object.__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:~object.__imul__, :meth:~object.__iand__, etc.) for the same purpose. It should not be evaluated in a boolean context. :data:!NotImplemented is the sole instance of the :data:types.NotImplementedType type.

.. note::

  When a binary (or in-place) method returns :data:`!NotImplemented` the
  interpreter will try the reflected operation on the other type (or some
  other fallback, depending on the operator).  If all attempts return
  :data:`!NotImplemented`, the interpreter will raise an appropriate exception.
  Incorrectly returning :data:`!NotImplemented` will result in a misleading
  error message or the :data:`!NotImplemented` value being returned to Python code.

  See :ref:`implementing-the-arithmetic-operations` for examples.

.. caution::

  :data:`!NotImplemented` and :exc:`!NotImplementedError` are not
  interchangeable. This constant should only be used as described
  above; see :exc:`NotImplementedError` for details on correct usage
  of the exception.

.. versionchanged:: 3.9 Evaluating :data:!NotImplemented in a boolean context was deprecated.

.. versionchanged:: 3.14 Evaluating :data:!NotImplemented in a boolean context now raises a :exc:TypeError. It previously evaluated to :const:True and emitted a :exc:DeprecationWarning since Python 3.9.

.. index:: single: ...; ellipsis literal .. data:: Ellipsis

The same as the ellipsis literal "...", an object frequently used to indicate that something is omitted. Assignment to Ellipsis is possible, but assignment to ... raises a :exc:SyntaxError. Ellipsis is the sole instance of the :data:types.EllipsisType type.

.. 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.

.. _site-consts:

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 accessed directly in the interactive interpreter or called as functions, raise :exc:SystemExit with the specified exit code.

.. data:: help :noindex:

Object that when printed, prints the message "Type help() for interactive help, or help(object) for help about object.", and when accessed directly in the interactive interpreter, invokes the built-in help system (see :func:help).

.. 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).