Separating elements from a list in Python depending on a condition

Question

I have a list of elements and want to separate the elements of the list by a certain condition.

A simple example is a list of numbers and i want to separate the odd from the even ones. For that could use the filter builtin like so:

def is_even(x):
    # ...

l = [0, 1, 2, 3, 4, 5, 6]

even = list(filter(is_even, l))
odd = list(filter(not is_even, l))

That is a bit error prone if the condition is a bit more complex, because i repeat myself twice in the filter functions. Is there a more elegant way to achieve this?

Answer 1

itertools has a recipe exactly for that:

from itertools import tee, filterfalse
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)

Usage:

odd, even = partition(is_even, l)

You can convert them to lists, but I suggest leaving them as iterators.