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

[Guido van Rossum]

I'm a bit too busy to look into this right now; I hope one or two more
rounds of feedback on the PEP will get it into a state where I can
review it more easily. Having a patch to go with it would be immensely
helpful as well (in fact I'd say that without a patch it's unlikely to
happen).

--Guido

2007/10/29, David A. Wheeler <dwheeler at dwheeler.com>:
> I think several postings have explained better than I have on why __cmp__ is still very valuable.  (See below.)
>
> Guido van Rossum posted earlier that he was willing to entertain a PEP to restore __cmp__, so I've attempted to create a draft PEP, posted here:
>   http://www.dwheeler.com/misc/pep-cmp.txt
> Please let me know if it makes sense.  Thanks.
>
> Greg Ewing stated "Why not provide a __richcmp__ method that directly connects
> with the corresponding type slot? All the comparisons eventually end up there anyway, so it seems like the right place to provide a one-stop comparison method in the 3.0 age."
> It _seems_ to me that this is the same as "__cmp__", and if so, let's just keep using the same name (there's nothing wrong with the name!).  But maybe I just don't understand the comment, so explanation welcome.
>
>
>
> --- David A. Wheeler
>
> ========================================
>
> Aahz:
> >From my perspective, the real use case for cmp() is when you want to do
> >a three-way comparison of a "large" object (for example, a Decimal
> >instance). You can store the result of cmp() and then do a separate
> >three-way branch.
>
> and reply to the note "I'm having troubles coming up with things where
> the *basic* operator is really a cmp-like function.", there were two replies..
>
>
> Guido van Rossum:
> >Here's one. When implementing the '<' operator on lists or tuples, you
> > really want to call the 'cmp' operator on the individual items,
> > because otherwise (if all you have is == and <) the algorithm becomes
> > something like "compare for equality until you've found the first pair
> > of items that are unequal; then compare those items again using < to
> > decide the final outcome". If you don't believe this, try to implement
> > this operation using only == or < without comparing any two items more
> > than once.
>
> and
>
> Greg Ewing:
> > Think of things like comparing a tuple. You need to work your
> > way along and recursively compare the elements. The decision
> > about when to stop always involves ==, whatever comparison
> > you're trying to do. So if e.g. you're doing <, then you have
> > to test each element first for <, and if that's false, test
> > it for ==. If the element is itself a tuple, it's doing this
> > on its elements too, etc., and things get very inefficient.
> >
> > If you have a single cmp operation that you can apply to the
> > elements, you only need to do it once for each element and it
> > gives you all the information you need.
>
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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)