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.

   2.3 新版功能.

True

   The true value of the "bool" type.

   2.3 新版功能.

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.

   在 2.4 版更改: 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. See also the "assert" statement.

注解: 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.

  在 2.7 版更改: 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.

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).
