Flattens nested list

flatten.py

def flatten(accumulator, lst):
    if len(lst) == 0:
        return accumulator
    elif type(lst[0]) is list:
        result = flatten([], lst[0])
        return flatten(accumulator + result, lst[1:])
    else:
        accumulator.append(lst[0])
        return flatten(accumulator, lst[1:])


print(flatten([], [[1, 2, 3], [4, 5, 6, 7], 8, 9, 10, [11, 12, 13]]))