On Fri, 25 Mar 2011 16:04:02 -0700, Carl Banks wrote: > On Mar 25, 3:06 pm, Steven D'Aprano <steve > +comp.lang.pyt...@pearwood.info> wrote: > > The reason Guido is considering re-introducing cmp is that somebody at > > Google approached him with a use-case where a key-based sort did not > > work. The use-case was that the user had masses of data, too much data > > for the added overhead of Decorate-Sort-Undecorate (which is what key > > does), but didn't care if it took a day or two to sort. > > > > So there is at least one use-case for preferring slowly sorting with a > > comparison function over key-based sorting. I asked if there any > > others. It seems not.
> 1. You asked for a specific kind of use case. Antoon gave you a use > case, you told him that wasn't the kind of use case you were asking for, > then you turn around and say "I guess there are no use cases" (without > the mentioning qualification). That is astonishing. Did you read my post before going on the attack? I know that there is a grand tradition of adversarial argument on Usenet, but I am not your enemy here. First you wrongly accuse me of stating that there are no use cases, without qualification, when the very text you quote has me giving a use- case. Nor did I say there are no use cases -- I said that I didn't think there were any *others*. You even quote me saying this! Then you go on to wrongly claim: > 3. You evidently also overlooked the use-case example posted on Python- > dev that you followed up to. No, I did not overlook it. I explicitly described it twice. Once in the very text you quoted above, and once early in the thread when I wrote: "Guido himself has mentioned one such good use for a comparison function when sorting. Use of a key function trades off memory for time, while sorting with a comparison function makes the opposite trade off, using more time for the sake of saving memory." > 2. I posted two use cases in this thread that fit your criteria, and you > followed up to that subthread so you most likely read them. Here they > are again so you won't overlook them this time: > > "You have are given an obscure string collating function implented in a > C library you don't have the source to." (Fits your criterion "can't be > done with key=".) I must admit I don't understand this use-case. What is a string collating function, and what does this have to do with sorting? I could try to guess from the plain English meaning of the words, but if I get it wrong, that wouldn't be fair to you. Perhaps an example would help. > "I'm sitting at an interactive session and I have a convenient cmp > function but no convenient key, and I care more about the four minutes > it'd take to whip up a clever key function or an adapter class than the > 0.2 seconds I'd save to on sorting time." (Fits your criterion > "performs really badly when done so".) "I can't be bothered to write a key function" is not equivalent to "I have a key function, but it is too slow to use". In any case, from Python 3.2 onwards, a key function is never more than one import and one function call away. (In 3.1, you're stuck writing your own adapter, but in 3.1 sort doesn't take a cmp function anyway, so either way you're stuck. The earliest it could be reintroduced, if at all, is in 3.3.) > Call me crazy, but you seem to be overlooking a lot of things in your > zeal to prove your point. And what point would that be? Carl, anybody could have asked the question I have asked. Anyone is free to summarize the best of the arguments -- or all of them -- and post them to python-dev. If you think I have some ulterior motive in doing this, please feel free to champion the cmp argument to python-dev instead. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
