Skip to content

Latest commit

 

History

History
89 lines (56 loc) · 2.78 KB

constants.rst

File metadata and controls

89 lines (56 loc) · 2.78 KB

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).