Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> writes: > If anyone has any use-cases for sorting with a comparison function that > either can't be written using a key function, or that perform really > badly when done so, this would be a good time to speak up.
We've had this discussion a couple times before. I remember an example involving comparing tree structures, that I thought couldn't be done with key= in a reasonable way, and that this was a reasonable real-world example. Raymond H then gave a way of encoding it with key= which looked ok at the time, but I decided sometime later that I think we both missed an error in that encoding, so I've been wanting to dig it out again and look more closely. key= can of course burn a lot more memory because of the DSU pattern (say you are sorting file handles according to the contents of the file, so with key= you might have to read N multi-megabyte files where with cmp you just pairwise-compare them until they differ, which is probably in the first few bytes). Finally I concocted an "infinite" example which we agreed is artificial: you are given a list of generators denoting real numbers, for example pi generates the infinite sequence 3,1,4,1,5,9... while e generates 2,7,1,8,... You can sort these with cmp but not with key. -- http://mail.python.org/mailman/listinfo/python-list
