Generating Primes

Using the Sieve of Eratosthenes, because I keep forgetting.

import timeit
NUM = 1000000

def generate_primes(n):
    nums = [i for i in xrange(n)]
    for i in xrange(2, n):
        if nums[i]:
            yield nums[i]
            for j in range(i*i, n, i):
                nums[j] = False

def gen_primes(n):
    primes = [True] * n
    primes[0] = primes[1] = False
    
    for (i, isprime) in enumerate(primes):
        if isprime:
            yield i
            for j in range(i*i, n, i):
                primes[j] = False

if __name__ == "__main__":
    # print timeit.timeit("list(gen_primes(NUM))", 
        # setup = "from __main__ import gen_primes, NUM", number=1)

    # print timeit.timeit("list(generate_primes(NUM))", 
    #   setup = "from __main__ import generate_primes, NUM", number=1)

gen_primes is from SO: 3939660

generate_primes is what I came up with. It is slower, and because Windows, I cannot find out why, yet. (Might be because xrange(n) is slower than enumerate(n) and / or [True] * n is faster than xrange(n))

The timeit print statements must be run one at a time, running both gives different results than if they are run individually.

Finding all primes less than a million takes ~0.4s on my machine (i5-661, 8 gigs of RAM)