Question

Say you have some list L and you want to split it into two lists based on some boolean function P. That is, you want one list of all the elements l where P(l) is true and another list where P(l) is false.

I can implement this in Python like so:

def multifilter(pred,seq):
    trues,falses = [],[]
    for x in seq:
        if pred(x):
            trues.append(x)
        else:
            falses.append(x)
    return trues,falses

My question: Is there a functional programming idiom that accomplishes this?

Answer 1

From itertools examples:

from itertools import tee, filterfalse
def partition(pred, iterable):
    t1, t2 = tee(iterable)
    return filterfalse(pred, t1), filter(pred, t2)