Sum of primes with sieve

def primes(n):
    yield 2
    length = (n+1)/2 - 1
    primes = [1] * length
    for i, is_prime in enumerate(primes):
        if is_prime:
            step = 2*i + 3
            yield step
            j = (step*step - 3)/2
            while j < length:
                primes[j] = 0
                j += step

print sum(primes(2*10**6))