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