4. Built-in Constants
A small number of constants live in the built-in namespace. They are:
The false value of the bool type.
The true value of the bool type.
The sole value of types.NoneType. None is frequently used to
represent the absence of a value, as when default arguments are not passed to a
Changed in version 2.4: Assignments to None are illegal and raise a SyntaxError.
Special value which can be returned by the “rich comparison” special methods
(__eq__(), __lt__(), and friends), to indicate that the comparison
is not implemented with respect to the other type.
Special value used in conjunction with extended slicing syntax.
This constant is true if Python was not started with an -O option.
See also the assert statement.
The names None and __debug__ cannot be reassigned
(assignments to them, even as an attribute name, raise SyntaxError),
so they can be considered “true” constants.
Changed in version 2.7: Assignments to __debug__ as an attribute became illegal.
4.1. 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).