判断一个数字是否是质数

import sys
def zhishu(a):
    if(a == 1):
        print('这即不是质数也不是合数')
    else:
        r = False
        for i in range(2,a):
            if(a % i == 0):
                r = True
                break
        if(r == True):
            print('这不是质数')
        else:
            print('这是质数')

b = sys.argv[1]
b1= int(b)
zhishu(b1)