Your code is doing something pretty different from the examples above. You're just calculating the nth fibonacci number, not creating an infinite fibonacci _seq

Your code is doing something pretty different from the examples above. You're just calculating the nth fibonacci number, not creating an infinite fibonacci _sequence_. Eliding the explicit recursive structure is the point of the exercise! Admittedly Python's facilities for doing that with some mutable state are pretty nice: from itertools import islice def fib(): a, b = 0, 1 while True: yield a a, b = b, a + b list(islice(fib(), 10)) == [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]