[Python-3000] Please re-add cmp to python 3000 (original) (raw)

Guido van Rossum guido at python.org
Mon Oct 29 21:26:16 CET 2007

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/)

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