array_flattener_with_deque.py

from collections import deque


def flatten(list_):
    return list(iflatten(list_))


def iflatten(list_):
    seen = set()
    list_ = deque(list_)
    while list_:
        item = list_.popleft()
        if isinstance(item, list):
            if id(item) in seen:
                raise TypeError
            else:
                seen.add(id(item))

            while item:
                list_.appendleft(item.pop())
        else:
            yield item

if __name__ == '__main__':
    print(flatten([[1, 2, [3]], [4, [5]]]))

array_flattener_with_recursion.py

def flatten(list_):
    return list(iflatten(list_))


def iflatten(list_, seen=None):
    if seen is None:
        seen = set()
    if id(list_) in seen:
        raise TypeError
    else:
        seen.add(id(list_))

    for i in list_:
        if isinstance(i, list):
            yield from iflatten(i, seen)
        else:
            yield i


if __name__ == '__main__':
    print(flatten([[1, 2, [3]], [4, [5]]]))