Highly optimized sieve of Erastosthenes in Python from https://codereview.stackexchange.com/questions/42420/sieve-of-eratosthenes-python

prime_sieve.py

import numpy

def sieve8(n):
    """Return an array of the primes below n."""
    prime = numpy.ones(n//3 + (n%6==2), dtype=numpy.bool)
    for i in range(3, int(n**.5) + 1, 3):
        if prime[i // 3]:
            p = (i + 1) | 1
            prime[       p*p//3     ::2*p] = False
            prime[p*(p-2*(i&1)+4)//3::2*p] = False
    result = (3 * prime.nonzero()[0] + 1) | 1
    result[0] = 3
    return numpy.r_[2,result]