3. Built-in Constants

A small number of constants live in the built-in namespace. They are:

False

The false value of thebool type. Assignments toFalseare illegal and raise aSyntaxError.

True

The true value of thebool type. Assignments toTrueare illegal and raise aSyntaxError.

None

The sole value of the typeNoneType.None is frequently used torepresent the absence of a value, as when default arguments are not passed to afunction. Assignments toNone are illegal and raise aSyntaxError.

NotImplemented

Special value which should be returned by the binary special methods(e.g.__eq__(),__lt__(),__add__(),__rsub__(),etc.) to indicate that the operation is not implemented with respect tothe other type; may be returned by the in-place binary special methods(e.g.__imul__(),__iand__(), etc.) for the same purpose.Its truth value is true.

Note

WhenNotImplemented is returned, the interpreter will then try thereflected operation on the other type, or some other fallback, dependingon the operator. If all attempted operations returnNotImplemented, theinterpreter will raise an appropriate exception.

SeeImplementing the arithmetic operationsfor more details.

Ellipsis

The same as.... Special value used mostly in conjunction with extendedslicing syntax for user-defined container data types.

__debug__

This constant is true if Python was not started with an-O option.See also theassert statement.

Note

The namesNone,False,True and__debug__cannot be reassigned (assignments to them, even as an attribute name, raiseSyntaxError), so they can be considered “true” constants.

3.1. Constants added by thesite module

Thesite module (which is imported automatically during startup, exceptif the-S command-line option is given) adds several constants to thebuilt-in namespace. They are useful for the interactive interpreter shell andshould 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 called, raiseSystemExit with thespecified exit code.

copyright
license
credits

Objects that when printed, print a message like “Type license() to see thefull license text”, and when called, display the corresponding text in apager-like fashion (one screen at a time).