Python Interlator Questions

class MultipleIterator:
    def __init__(self, stop, divisor):
        self.current = 0
        self.stop = stop
        self.divisor = divisor

    def __iter__(self):
        return self

    def __next__(self):
        self.current += 1
        if self.current < self.stop:
            if self.current % self.divisor == 0:
                return self.current
        else:
            raise StopIteration

for i in MultipleIterator(20, 3):
    print(i, end=' ')

# Execution result
None None 3 None None 6 None None 9 None None 12 None None 15 None None 18 None

# When I do it in a different way,
a = MultipleIterator(20, 3).__iter__()
a.__next__() # Nothing comes out
a.__next__() # Nothing comes out
a.__next__() #3 Output
3