in fact to reiterate have a look at the awesome fibonacci example showing power of generators [1]:

    def fibonacci(max):
        a, b = 0, 1       
        while a < max:
            yield a       
            a, b = b, a+b

To run the function do:

    >>> for n in fibonacci(1000): 
    ...     print n,              
    0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987

[1] http://diveintopython.org/dynamic_functions/stage6.html