A small number of constants live in the built-in namespace. They are:
The false value of the
booltype. Assignments to
Falseare illegal and raise a
The true value of the
booltype. Assignments to
Trueare illegal and raise a
The sole value of the type
Noneis frequently used to represent the absence of a value, as when default arguments are not passed to a function. Assignments to
Noneare illegal and raise a
Special value which should be returned by the binary special methods (e.g.
__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.
__iand__(), etc.) for the same purpose. It should not be evaluated in a boolean context.
When a binary (or in-place) method returns
NotImplementedthe 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
NotImplementedwill result in a misleading error message or the
NotImplementedvalue being returned to Python code.
See Implementing the arithmetic operations for examples.
NotImplementedare not interchangeable, even though they have similar names and purposes. See
NotImplementedErrorfor details on when to use it.
Changed in version 3.9: Evaluating
NotImplementedin a boolean context is deprecated. While it currently evaluates as true, it will emit a
DeprecationWarning. It will raise a
TypeErrorin a future version of Python.
The same as the ellipsis literal “
...”. Special value used mostly in conjunction with extended slicing syntax for user-defined container data types.
This constant is true if Python was not started with an
-Ooption. See also the
cannot be reassigned (assignments to them, even as an attribute name, raise
SyntaxError), so they can be considered “true” constants.
Constants added by the
site module (which is imported automatically during startup, except
-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.
Objects that when printed, print a message like “Use quit() or Ctrl-D (i.e. EOF) to exit”, and when called, raise
SystemExitwith the specified exit code.
Objects that when printed or called, print the text of copyright or credits, respectively.
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).