[issue4395] Document auto __ne__ generation; provide a use case for non-trivial __ne__

[Michael K. Edwards]

Michael K. Edwards <m.k.edwards at gmail.com> added the comment:

The implementation you are looking for is in object_richcompare, in
http://svn.python.org/projects/python/branches/py3k/Objects/typeobject.c
.  It would be most accurate to say something like:

    The "object" base class, from which all user-defined classes
inherit, provides a single "rich comparison" method to which all of the
comparison operators (__eq__, __ne__, __lt__, __le__, __ge__, __gt__)
map.  This method returns a non-trivial value (i. e., something other
than NotImplemented) in only two cases:
  * When called as __eq__, if the two objects are identical, this method
returns True.  (If they are not identical, it returns NotImplemented so
that the other object's implementation of __eq__ gets a chance to return
True.)
  * When called as __ne__, it calls the equivalent of "self == other";
if this returns a non-trivial value X, then it returns !X (which is
always either True or False).

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