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class Fib:
    '''iterator that yields numbers in the Fibonacci sequence'''

    def __init__(self, max):
        self.max = max
        self.a = 0
        self.b = 1

    def __iter__(self):
        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

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>>> import fibonacci2
>>> fib = fibonacci2.Fib(100)
>>> fib.next()
0
>>> fib.next()
1
>>> fib.next()
1
>>> fib.next()
2
>>> fib.next()
3
>>> fib.next()
5
>>> fib.next()
8
>>> fib.next()
13
>>> fib.next()
21
>>> fib.next()
34
>>> fib.next()
55
>>> fib.next()
89
>>> fib.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "fibonacci2.py", line 15, in next
    raise StopIteration
StopIteration