Python Built in Constants

title: Python Built in Constants


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

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