[Python-3000] Please re-add __cmp__ to python 3000
David A. Wheeler
dwheeler at dwheeler.com
Mon Oct 29 19:34:03 CET 2007
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:
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
>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
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.
> 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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