Python defining an iterator class, failed with "iter() returned non-iterator of type 'Fib'"

Question

I'm using python 2.7 and ipython2.7. In ipython I tried:

class Fib(object):
    def __init__(self, max):
        super(Fib, self).__init__()
        self.max = max

    def __iter__(self):
        self.a = 0
        self.b = 1
        return self

    def __next__(self):
        fib = self.a
        if fib > self.max:
            raise StopIteration
        self.a, self.b = self.b, self.a + self.b
        return fib

def main():
    fib = Fib(100)
    for i in fib:
        print i

if __name__ == '__main__':
    main()

Well it reports an error:

TypeError Traceback (most recent call last) in () 22 23 if name == 'main': ---> 24 main() 25

<ipython-input-21-f10bd2d06666> in main()
    18 def main():
    19     fib = Fib(100)
---> 20     for i in fib:
    21         print i
    22

TypeError: iter() returned non-iterator of type 'Fib'

This piece of code is actually from internet. The gramma seems OK to me, but how the problem happens?

Thanks.

Answer 1

def __next__(self) is for Python 3

For Python 2 you need to add method next()

This code will work under both Python 3 and Python 2:

class Fib(object):
    def __init__(self, max):
        super(Fib, self).__init__()
        self.max = max

    def __iter__(self):
        self.a = 0
        self.b = 1
        return self

    def __next__(self):
        fib = self.a
        if fib > self.max:
            raise StopIteration
        self.a, self.b = self.b, self.a + self.b
        return fib

    def next(self):
        return self.__next__()


def main():
    fib = Fib(100)
    for i in fib:
        print(i)

if __name__ == '__main__':
    main()

Answer 2

That's python 3 code where the iterator protocol uses the __next__ method. The best fix is to start using python 3. But if you need python 2, just change __next__ to next. The reason for the change was to make sure magic methods such as next conform to the __ naming convention.