Python 3.6.5 Documentation >  Built-in Constants

Built-in Constants
******************

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

False

The false value of the "bool" type. Assignments to "False" are
illegal and raise a "SyntaxError".

True

The true value of the "bool" type. Assignments to "True" are
illegal and 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" are illegal and
raise a "SyntaxError".

NotImplemented

Special value which should be returned by the binary special
methods (e.g. "__eq__()", "__lt__()", "__add__()", "__rsub__()",
etc.) to indicate that the operation is not implemented with
respect to the other type; may be returned by the in-place binary
special methods (e.g. "__imul__()", "__iand__()", etc.) for the
same purpose. Its truth value is true.

Note: When a binary (or in-place) method returns "NotImplemented"
the interpreter will try the reflected operation on the other
type (or some other fallback, depending on the operator). If all
attempts return "NotImplemented", the interpreter will raise an
appropriate exception. Incorrectly returning "NotImplemented"
will result in a misleading error message or the "NotImplemented"
value being returned to Python code.See Implementing the
arithmetic operations for examples.

Note: "NotImplementedError" and "NotImplemented" are not
interchangeable, even though they have similar names and
purposes. See "NotImplementedError" for details on when to use
it.

Ellipsis

The same as "...". Special value used mostly in conjunction with
extended slicing syntax for user-defined container data types.

__debug__

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

Note: The names "None", "False", "True" and "__debug__" cannot be
reassigned (assignments to them, even as an attribute name, raise
"SyntaxError"), so they can be considered “true” constants.

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.

copyright
credits

Objects that when printed or called, print the text of copyright or
credits, respectively.

license

Object that when printed, prints the message “Type license() to see
the full license text”, and when called, displays the full license
text in a pager-like fashion (one screen at a time).