[Python-3000] Please re-add __cmp__ to python 3000

[Greg Ewing]

Adam Olsen wrote:
>             for a, b in zip(self.data, other.data):
>                 result = richcmp(a, b, ordered)
>                 if result:
>                     return result

That can't be right, because there are *three* possible
results you need to be able to distinguish from comparing
a pair of elements: "stop and return True", "stop and
return False", and "keep going". There's no way you can
get that out of a boolean return value.

Maybe what we need to do is enhance __cmp__ so that
it has *four* possible return values: -1, 0, 1 and
UnequalButNotOrdered.

The scheme for handling a comparison 'op' between
two values 'a' and 'b' would then be:

1) Try a.__richcmp__(op, b) and vice versa. If either
    of these produces a result, return it.

2) Try a.__cmp__(b) and vice versa. If either of these
    produces a result, then

    a) If the result is -1, 0 or 1, return an appropriate
       value based on the operation.

    b) If the result is UnequalButNotOrdered, and the
       operation is == or !=, return an appropriate
       value.

    c) Otherwise, raise an exception.

The pattern for comparing sequences would become:

   def __cmp__(self, other):
     for a, b in zip(self.items, other.items):
       result = cmp(a, b)
       if result != 0:
         return result
      return 0

Which is actually the same as it is now, with an added
bit of It Just Works behaviour: if any of the element
comparisons gives UnequalButNotOrdered, then the whole
sequence gets reported as such.

--
Greg