Remember, if an argument is iterable, it means that you can call only iter on it, and then call next on the value iter returns (for loops do this automatically). There is no guarantee you can call len on the iterable or index/slice it. You may not copy all the values of an iterable into a list (or any other data structure) so that you can perform these operations (that is not in the spirit of the assignment, and some iterables could produce an infinite number of values, so such copying is impossible). You may create local data structures storing as many values as the arguments or the result that the function returns, but not all the values in the iterable. In fact, your code must work on infinite iterables (see the primes and nth generators, which work together for testing).