Is there a way to "fork" a list in two based on a condition [duplicate]

Question

I have seen a pattern repeated a couple times in my team's code, it looks like this

numbers = [1, 2, 3, 4]
even_numbers = [n for n in numbers if n % 2 == 0]
odd_numbers = [n for n in numbers if n % 2 != 0]

I was wondering if there is a function somewhere (I have looked around but haven't been able to find it) that would do something like this

numbers = [1, 2, 3, 4]
even_numbers, odd_numbers = fork(numbers, lambda x: x % 2 == 0)

So, this function I am looking for, would receive an iterable and a function, and return two lists, one would be the values that match a provided condition, and the other would be the ones that didn't.

Is there something around python's standard library that achieves this?

Answer 1

I usually call this sift, but partition is fine too.

Another, itertools-less implementation might be

def sift(iterable, predicate):
    t = []
    f = []
    for value in iterable:
        (t if predicate(value) else f).append(value)
    return (t, f)

even, odd = sift([1, 2, 3, 4, 5], lambda x: x % 2 == 0)

EDIT: for a slightly more complex implementation that is about 30% faster (on my Python installation anyway):

def sift2(iterable, predicate):
    t = []
    f = []
    ta = t.append
    fa = f.append
    for value in iterable:
        (ta if predicate(value) else fa)(value)
    return (t, f)