Michael Hoffman <[EMAIL PROTECTED]> wrote: > Steven D'Aprano wrote: > > On Wed, 02 May 2007 06:10:54 +0000, Tim Roberts wrote: > > >> I've tended to favor the "Schwarzian transform" (decorate-sort-undecorate) > >> because of that. > > > > That's what the key= argument does. cmp= is slow because the comparison > > function is called for EVERY comparison. The key= function is only called > > once per element. > > Right. Using sort(key=keyfunc) is supposed to be faster than > decorate-sort-undecorate. And I think it is clearer too.
Right, and speed comparisons are pretty easy (just make sure to copy the list within the loop to compare like with like:-)...: brain:~ alex$ python -mtimeit -s'L=range(-3,5)' 'X=L[:]; X.sort(lambda x, y: cmp(abs(x), abs(y)))' 100000 loops, best of 3: 13 usec per loop brain:~ alex$ python -mtimeit -s'L=range(-3,5)' 'X=[(abs(x),x) for x in L]; X.sort(); X=[x for _,x in X]' 100000 loops, best of 3: 7.85 usec per loop brain:~ alex$ python -mtimeit -s'L=range(-3,5)' 'X=L[:]; X.sort(key=abs)' 100000 loops, best of 3: 4.23 usec per loop The "intrinsic" DSU done by key= is clearly superior on all scores. The semantics are subtly different: it guarantees to never compare anything _except_ the key (because that's typically what one wants), so, make sure they key includes everything you may ever want compared. Alex -- http://mail.python.org/mailman/listinfo/python-list
