flatten_list.py

a = [1, [2, 2, 2], 4]

def flat_list(x):
    return iter(x)

def make_iter(data):
    for i in range(len(data)):
        if type(data[i]) == list:
            data[i] = make_iter(data[i])
            data[i] = iter(data[i])
    return data

def cover(func, in_data):
    res = func(iter(make_iter(in_data)))
    assert hasattr(res, '__iter__'), "your function should return the iterator object"
    assert hasattr(res, '__next__'), "your function should return the iterator object"
    return list(res)

z = cover(flat_list, a)

print(list(z))
print(list(z[1]))