4. Built-in Constants

A small number of constants live in the built-in namespace. They are:

False

The false value of the bool type.

New in version 2.3:

New in version 2.3.

True

The true value of the bool type.

New in version 2.3:

New in version 2.3.

None

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 function.

Changed in version 2.4:

Changed in version 2.4: Assignments to None are illegal and raise a SyntaxError.

NotImplemented

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.

Ellipsis

Special value used in conjunction with extended slicing syntax.

__debug__

This constant is true if Python was not started with an -O option. It cannot be reassigned. See also the assert statement.

Note

The name None cannot be reassigned (assignments to it, even as an attribute name, raise SyntaxError), so it can be considered a “true” constant.

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.

quit([code=None])
exit([code=None])

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.

license
credits

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).

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