sum.py

def sum(xs):
    def aux(xs):
        try:
            x = next(xs)
            return x + aux(xs)
        except StopIteration:
            return 0

    return aux(iter(xs))


assert sum([]) == 0
assert sum((1, 2, 3)) == 6
assert sum(range(10)) == 45


def fancy_sum(xs):
    from functools import reduce
    from operator import add

    return reduce(add, xs, 0)


assert fancy_sum([]) == 0
assert fancy_sum((1, 2, 3)) == 6
assert fancy_sum(range(10)) == 45