Incremental Sieve of Eratosthenes

def get_primes_to(n):
    D, q = {}, 2
    while q <= n:
        if q not in D:
            yield q
            D[q * q] = [q]
        else:
            for p in D[q]:
                D.setdefault(p + q, []).append(p)
            del D[q]
        q += 1