--- title: Python Built in Constants --- 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