m.py

username = "python"
userpassword = "123456"
while True:
    x=input("请输入账号：")
    y=input("请输入密码：")
    if x==username and y==userpassword:
        print("密码正确")
        break
    if x != username:
        input("用户名不存在")
        continue
    if y != userpassword:
        input("密码错误")