Adds primes into a file that lists primes. (Absurdly fast single-thread prime finder.)

prim.py

#!/usr/bin/env python

import time
import os.path
import atexit
import argparse


def write_primes(filename, known_primes):
    """Writes all known primes from RAM to disk"""
    f = open(filename, "w")
    write = lambda p: f.write("%i\n" % p)
    [write(p) for p in sorted(list(set(known_primes)))]
    f.close()


def read_primes(filename):
    """Loads all saved numbers into RAM"""
    if os.path.exists(filename):
        f = open(filename, "r")
        primes = [int(l) for l in f.readlines()]
        f.close()
        if len(primes) > 3:
            return primes
    return [2, 3, 5]


def check_prime(n, known_primes):
    """Lazily checks whether n is prime using known primes"""
    for p in known_primes:
        if p * p >= n:
            return True
        if n % p == 0:
            return False
    return True


def find_primes(known_primes, limit=500000):
    n = known_primes[-1]
    kp_app = known_primes.append
    while n < args.top:
        n += 2
        if check_prime(n, known_primes):
            kp_app(n)


if __name__ == "__main__":
    app = argparse.ArgumentParser(description="Prime number finder")
    app.add_argument("listfile", default="list.txt", type=str,
                     help="give it some primes")
    app.add_argument("--top", "-t", default=500000, type=long,
                     help="greates number to check for being prime")
    args = app.parse_args()

    known_primes = read_primes(args.listfile)
    atexit.register(write_primes, args.listfile, known_primes)

    t = time.time()
    find_primes(known_primes, args.top)
    print "It took %.2fs for %i prime numbers." % (time.time() - t, len(known_primes))

Calculates all prime numbers from 1 to 500000 in 1.67 seconds on a single 1.67 GHz core.