Tom Switzer <thomas.swit...@gmail.com> added the comment: I am not sure I understand the reasoning behind removing the cmp parameter (and agree with Lea Wiemann). Trying to wedge a proper comparison into the key parameter is clumsy and unreadable (as can be seen in the 2to3 example above). The intrinsic ordering on objects does not necessarily match up with the way you want to sort them. For example, a natural intrinsic order on 2 points in 2d is lexicographical, however you often want to sort by angular order relative to some other point instead. Clearly this can never be put in __cmp__ or __lt__, because the sorted order is relative to some other unknown point. Trying to do this with the key function doesn't make sense; it would not be clear you are sorting by angular order and you'd have to instantiate a bunch of wrapper objects just to do basic sorting. Another quick example would be sorting hyperplanes by intersection on a ray. Sorting points along a direction given by a vector.
I understand removing redundant features from a language, but I just can't see how key replaces this functionality in a readable or efficient way. This highlights an important class of cases (since it was mentioned that none could be thought of) in which we wish to make comparisons between values where a comparison (<, > or ==) is more numerically sound, more efficient, or the only option (perhaps the ordering is defined explicitly) then computing the exact values (eg. angle). As far as it seems, the only way to do this with key is by following the example given and creating a class solely to wrap each object that overrides __cmp__, which is certainly non-obvious (ie. there is no one, obvious way to do it). ---------- nosy: +tixxit _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue1771> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
