mirror of
https://github.com/python/cpython.git
synced 2025-11-03 19:34:08 +00:00
89 lines
2.8 KiB
ReStructuredText
89 lines
2.8 KiB
ReStructuredText
.. _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).
|
|
|