python equivalent of filter() getting two output lists (i.e. partition of a list)

Question

Let's say I have a list, and a filtering function. Using something like

>>> filter(lambda x: x > 10, [1,4,12,7,42])
[12, 42]

I can get the elements matching the criterion. Is there a function I could use that would output two lists, one of elements matching, one of the remaining elements? I could call the filter() function twice, but that's kinda ugly :)

Edit: the order of elements should be conserved, and I may have identical elements multiple times.

Answer 1

Try this:

def partition(pred, iterable):
    trues = []
    falses = []
    for item in iterable:
        if pred(item):
            trues.append(item)
        else:
            falses.append(item)
    return trues, falses

Usage:

>>> trues, falses = partition(lambda x: x > 10, [1,4,12,7,42])
>>> trues
[12, 42]
>>> falses
[1, 4, 7]

There is also an implementation suggestion in itertools recipes:

from itertools import filterfalse, tee

def partition(pred, iterable):
    'Use a predicate to partition entries into false entries and true entries'
    # partition(is_odd, range(10)) --> 0 2 4 6 8   and  1 3 5 7 9
    t1, t2 = tee(iterable)
    return filterfalse(pred, t1), filter(pred, t2)

The recipe comes from the Python 3.x documentation. In Python 2.x filterfalse is called ifilterfalse.