Question

Advice on optimizing this program for summing primes:

Answer 1

import math
def SumPrimesBelow(num):
s=0
composites=set([])
for i in range(2,num+1):
sieve=[p*i for p in range(2,int(num/i+1))]
composites.update(sieve)
num = set(range(2,num+1))
primes_below_num = num.difference(composites)
for prime in primes_below_num:
s+=prime
return s

print SumPrimesBelow(2000000)