weird_iterator.py

class BadIterable:
    def __iter__(self):
        class Iterator:
            def __init__(self):
                self.n = 0
            def __next__(self):
                return self.n
            def __iter__(self):
                return self
            def advance(self, by):
                self.n += by
            def previous(self, by):
                self.n -= by
        return Iterator()

it = iter(BadIterable())
for n in it:
    if n % 3 == 0:
        it.previous(3)
    print(n)
    it.advance(2)

    if n > 100:
        break
        
# Prints
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# -1
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