seive.py

def iprimes_upto(limit):
    is_prime = [False] * 2 + [True] * (limit - 1)
    for n in xrange(int(limit**0.5 + 1.5)): # stop at ``sqrt(limit)``
        if is_prime[n]:
            for i in range(n * n, limit + 1, n): # start at ``n`` squared
                is_prime[i] = False
    for i in xrange(limit + 1):
        if is_prime[i]: yield i