I'd take a 2-pass approach, separating evaluation of the predicate from filtering the list:
def partition(pred, iterable): xs = list(zip(map(pred, iterable), iterable)) return [x[1] for x in xs if x[0]], [x[1] for x in xs if not x[0]]What's nice about this, performance-wise (in addition to evaluating pred only once on each member of iterable), is that it moves a lot of logic out of the interpreter and into highly-optimized iteration and mapping code. This can speed up iteration over long iterables, as described in this answer.
Expressivity-wise, it takes advantage of expressive idioms like comprehensions and mapping.