[Python-3000] Total ordering and __cmp__

[Steven Bethard]

On 3/20/07, Collin Winter <collinw at gmail.com> wrote:
> Quoting from the commit message for r51533, which removed the default ordering:
>
> """
> A general problem with getting lots of these tests to pass is
> the reality that for object types that have a natural total ordering,
> implementing __cmp__ is much more convenient than implementing
> __eq__, __ne__, __lt__, and so on.  Should we go back to allowing
> __cmp__ to provide a total ordering?  Should we provide some other
> way to implement rich comparison with a single method override?
> Alex proposed a __key__() method

I've used a __key__() method quite successfully in my own code.  Maybe
we should provide a mixin like::

    class KeyedComparisonMixin(object):
        def __eq__(self, other):
            return self.__key__() == other.__key__()
        def __ne__(self, other):
            return self.__key__() != other.__key__()
        def __lt__(self, other):
            return self.__key__() < other.__key__()
        def __le__(self, other):
            return self.__key__() <= other.__key__()
        def __gt__(self, other):
            return self.__key__() > other.__key__()
        def __ge__(self, other):
            return self.__key__() >= other.__key__()

That way, any classes that needed __cmp__-like behavior could request
it explicitly by using the mixin.  (If people really want it, we could
define a __cmp__-based mixin instead, but I've *always* found the
__key__-based mixin to be a better approach in my own code.)

STeVe
-- 
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