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python-3.7.4-docs-html/_sources/library/constants.rst.txt

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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.
+
+
+.. index:: single: ...; ellipsis literal
+.. data:: Ellipsis
+
+   The same as the ellipsis literal "``...``".  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
+          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).
+