How would you test this? The Miller-Rabin Primality test? For large numbers, iterating through candidate prime divisors is pricey. 

How would you implements this? Sieve of Eratosthenes is really fun to show students as a Python generator function. But the cached primes DO grow unboundedly as you utilize the generator. Wheel factorization as first pass? Do you cached the first N primes so the each instance of iterator can provide initial elements much faster? What is N?