pysieve.py

#!/usr/bin/env python3

import sys

def main():
    start, end = 0, int(sys.argv[1])
    primelist = [True] * (end+1)                # populate array with Trues from 0 
    primelist[0], primelist[1] = False, False   # 0 and 1 are not primes

    for i in range(2, int(end ** 0.5)+1):       # the sieve
        if primelist[i]:
            j = i ** 2 
            while j < end+1:
                primelist[j] = False
                j += i
    count = 0

    for (n, prime) in enumerate(primelist):     # print any numbers marked as True
        if prime:
            count += 1
    print(count)

main()