class Iterator:
    def __init__(self, origin_list):
        self._ref = origin_list
        self.n = -1

    def __next__(self):
        if self.n < len(self._ref) - 1:
            self.n += 1
            return self._ref[self.n]
        raise StopIteration()


class A:
    def __init__(self):
        self.a = [1, 2, 3, 4]

    def __iter__(self):
        return Iterator(self.a)


a = A()
for el in a:
    print(el)