A small number of constants live in the built-in namespace. They are:
False
The false value of the
type. Assignments to False are illegal and raise a
.
True
The true value of the
type. Assignments to True are illegal and raise a
.
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
. None is the sole instance of the
type.
NotImplemented
A special value which should be returned by the binary special methods (e.g.
,
,
,
, 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.
,
, etc.) for the same purpose. It should not be evaluated in a boolean context. NotImplemented is the sole instance of the
type.
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
Implementing the arithmetic operations
for examples.
Caution
NotImplemented and NotImplementedError are not interchangeable. This constant should only be used as described above; see
for details on correct usage of the exception.
Changed in version 3.9: Evaluating NotImplemented in a boolean context was deprecated.
Changed in version 3.14: Evaluating NotImplemented in a boolean context now raises a
. It previously evaluated to
and emitted a
since Python 3.9.
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
. Ellipsis is the sole instance of the
type.
__debug__
This constant is true if Python was not started with an
option. See also the
statement.
Note
The names
,
,
and
cannot be reassigned (assignments to them, even as an attribute name, raise
), so they can be considered “true” constants.
Constants added by the
module
The
module (which is imported automatically during startup, except if the
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.
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
with the specified exit code.
helpObject 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
).
copyright
credits
Objects that when printed or called, print the text of copyright or credits, respectively.
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).