寻找某个大数的最大质数

# -*- coding: utf-8 -*-
#寻找某个大数的最大质数

# 判断是否为质数，是质数返回1，否则返回0
def is_prime(num):
    limit = int(pow(num, 0.5))
    i = 2

    while 1:
        if num % i == 0 or i > limit:
            break
        i += 1

    if i > limit:
        return 1
    return 0

number = 600851475143
prime = 2

# 主程序
while not is_prime(number):
    while number % prime == 0:
        number /= prime
    prime += 1

print number