Alex Martelli wrote: > Using sum on lists is DEFINITELY slow -- avoid it like the plague. > > If you have a list of lists LOL, DON'T use sum(LOL, []), but rather > > [x for x in y for y in LOL] Should be >>> lol = [[1,2],[3,4]] >>> [x for y in lol for x in y] [1, 2, 3, 4] The outer loop comes first. Kent