title: Python Built in Constants
Three commonly used built-in constants:
True: The true value of the bool type. Assignments to
Trueraise a SyntaxError.
False: The false value of the bool type. Assignments to
Falseraise a SyntaxError.
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
Noneraise a SyntaxError.
Other built-in constants:
NotImplemented: Special value which should be returned by the binary special methods, such as
__rsub__(), etc.) to indicate that the operation is not implemented with respect to the other type.
Ellipsis: Special value used mostly in conjunction with extended slicing syntax for user-defined container data types.
__debug__: True if Python was not started with an -o option.
Constants added by the site module
The site module (which is imported automatically during startup, except if the -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 SystemExit with the specified exit code:
Objects that when printed, print a message like “Type license() to see the full license text”, and when called, display the corresponding text in a pager-like fashion (one screen at a time):