prime_sieve_less_memory.py

def gen_primes():
    """Generate an infinite sequence of prime numbers."""
    D = {}
    q = 2
    S = 4
    r = 2
    while True:
        if q not in D:
            if q == S:
                S = S + r
                D.setdefault(S, []).append(r)
                r = r + 1
                S = S + r
            else:
                yield q
        else:
            for p in D[q]:
                D.setdefault(p + q, []).append(p)
            del D[q]
            if q == S:
                S = S + r
                r = r + 1
                S = S + r
        q += 1