Recursive Permutation in Python

perm.py

def permutation(xs):
    if len(xs) == 1:
        return [xs]
    ret = []
    for x in xs:
        for y in permutation(filter(lambda a: a != x, xs)):
            ret.append([x] + y)
    return ret

def perm(xs):
    if len(xs) == 1:
        return [xs]
    else:
        return [([x] + y) for x in xs for y in perm(filter(lambda a: a != x, xs))]

print perm([1,2,3, 4])
print permutation([1,2,3,4])