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

When a binary (or in-place) method returnsNotImplemented theinterpreter will try the reflected operation on the other type (or someother fallback, depending on the operator). If all attempts returnNotImplemented, the interpreter will raise an appropriate exception.Incorrectly returningNotImplemented will result in a misleadingerror message or theNotImplemented value being returned to Python code.

SeeImplementing the arithmetic operations for examples.

Note

NotImplementedError andNotImplemented are not interchangeable,even though they have similar names and purposes.SeeNotImplementedError for details on when to use it.

Ellipsis

The same as the ellipsis literal “...”. Special value used mostly in conjunctionwith extended slicing 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.

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
credits

Objects that when printed or called, print the text of copyright orcredits, respectively.

license

Object that when printed, prints the message “Type license() to see thefull license text”, and when called, displays the full license text in apager-like fashion (one screen at a time).