class Squares:
    def __init__(self, n):
        self.n = n

    def __iter__(self):
        return self.SquaresIterator(self.n)

    class SquaresIterator:
        def __init__(self, n):
            self.n = n
            self.i = 0

        def __iter__(self):
            return self

        def __next__(self):
            if self.i >= self.n:
                raise StopIteration

            result = self.i ** 2
            self.i += 1
            return result

sq_class = Squares(5)
list(sq_class)
[0, 1, 4, 9, 16]
# calling again will create a new iterator
list(sq_class)
[0, 1, 4, 9, 16]

def squares(n):
    for i in range(n):
        yield i ** 2

sq_gen = squares(5)
list(sq_gen)
[0, 1, 4, 9, 16]
# calling again will return empty [] is OK, generator exhausted.
list(sq_gen)
[]

def Squares(n):
    def inner():
        return squares_gen(n)

    def squares_gen(n):
        for i in range(n):
            yield i ** 2

    return inner

sq_closure = Squares(5)
list(sq_closure())
[0, 1, 4, 9, 16]
# calling again the inner will create a new generator so it is not exhausted
list(sq_closure())
[0, 1, 4, 9, 16]

def Squares(n):
    def inner():
        return squares_gen(n)

    def squares_gen(n):
        for i in range(n):
            yield i ** 2

    inner.__iter__ = lambda : squares_gen(n)
    return inner

sq_closure = Squares(5)
list(sq_closure)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-13-beb02e61ccfb> in <module>
----> 1 for i in sq:
      2     print(i)

TypeError: 'function' object is not iterable