An Introduction to Python Built in Constants: True, False, and None in Python

Three commonly used built-in constants:

  • True : The true value of the bool type. Assignments to True raise a SyntaxError .
  • False : The false value of the bool type. Assignments to False raise 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 None raise a SyntaxError .

Other built-in constants:

  • NotImplemented : Special value which should be returned by the binary special methods, such as __eg__() , __add__() , __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:

  • quit(code=None)
  • exit(code=None)

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):

  • copyright
  • license
  • credits