class list2(list):
    def __iter__(self):
        return list2iterator(self)

class list2iterator:
    def __init__(self, X):
        self._X = X
        self._pointer = -1

    def __next__(self):
        self._pointer += 1
        if self._pointer == len(self._X):
            self._pointer = -1
            raise StopIteration
        return self._X[self._pointer]

>>> from test_iter import list2
>>> X=list2([1,2,3])
>>> [x for x in X]
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    [x for x in X]
                ^
TypeError: 'list2iterator' object is not iterable

>>> from test_iter import list2
>>> X=list2([1,2,3])
>>> [x for x in X]
[1, 2, 3]