fg.py

username = "python"
userpassword = "123456"
while True:
    name = input("请输入用户名:")
    password = input("请输入密码:")
    if name == username and password == userpassword:
        print("登录成功!")
        break
    elif name!= username:
        print("用户名不存在，请重新输入")       
    else :
        print("密码不存在，请重新输入")   
    print("用户名和密码错误!请重新输入")
print("欢迎来到贝尔编程!")