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Python 3: An interactive deep dive

A Fibonacci Iterator

Explore how to build a Fibonacci iterator from scratch in Python by defining a class with __init__, __iter__, and __next__ methods. Understand how iterators work with for loops and the role of the StopIteration exception in terminating iteration gracefully.

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Now you’re ready to learn how to build an iterator. An iterator is just a class that defines an __iter__() method.

Python 3.5
class Fib:                                        #①
    def __init__(self, max):                      #②
        self.max = max
    def __iter__(self):                           #③
        self.a = 0
        self.b = 1
        return self
    def __next__(self):                           #④
        fib = self.a
        if fib > self.max:
            raise StopIteration                   #⑤
        self.a, self.b = self.b, self.a + self.b
        return fib                                #⑥

① To build an iterator from scratch, Fib needs to be a class, not a function.

② “Calling” Fib(max) is really creating an instance of this class and calling its __init__() method with max. The __init__() method saves the maximum value as an instance variable so other methods can refer to it later.

③ The __iter__() method is called whenever someone calls iter(fib). (As you’ll see in a minute, a for loop will call this automatically, but you can also call it yourself manually.) After performing beginning-of-iteration initialization (in this case, resetting self.a and self.b, our two counters), the ...