Python 3 __cmp__ semantic change?
Duncan Booth
duncan.booth at invalid.invalid
Fri Nov 21 04:09:05 EST 2008
Johannes Bauer <dfnsonfsduifb at gmx.de> wrote:
> Seems it was removed on purpose - I'm sure there was a good reason for
> that, but may I ask why? Instead of the sleek __cmp__ function I had
> earlier, I now have code like:
>
>
> def __lt__(self, other):
> return self.__cmp__(other) < 0
>
> def __le__(self, other):
> return self.__cmp__(other) < 0
I hope you actually have <= here.
>
> def __gt__(self, other):
> return self.__cmp__(other) > 0
>
> def __ge__(self, other):
> return self.__cmp__(other) >= 0
>
> Does anyone know the reason why __cmp__ was discarded?
I think it was because __cmp__ was the backward compatible fallback for
the newer rich comparison methods and Python 3 cleans up a lot of stuff
left in just for backward compatibility. In this case it is a cleanup
too far as in most cases (i.e. those cases where you don't need the full
complexity of the rich comparisons) __cmp__ is a much simpler solution.
See http://mail.python.org/pipermail/python-dev/2003-March/034073.html
for Guido's original thoughts. Also, once upon a time pep-3000 referred
to the removal of __cmp__ but I can't find it in any of the current
peps. See
http://mail.python.org/pipermail/python-checkins/2004-August/042959.html
and
http://mail.python.org/pipermail/python-checkins/2004-August/042972.html
where the reference to removal of __cmp__ became "Comparisons other than
``==`` and ``!=`` between disparate types will raise an exception unless
explicitly supported by the type" and the reference to Guido's email
about removing __cmp__ was also removed.
--
Duncan Booth http://kupuguy.blogspot.com
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