3. 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 the ellipsis literal “"..."”.  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.


3.1. 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.

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).
