Python: split a list based on a condition?

Question

What\'s the best way, both aesthetically and from a performance perspective, to split a list of items into multiple lists based on a conditional? The equivalent of:

Answer 1

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.