i=3
username = "python"
userpassword = "123456"
while True:
    if i>0:
        name = input("请输入用户名:")
        password = input("请输入密码:")
        i=i-1
        if name == username and password == userpassword:
            print("登录成功!")
            print("欢迎来到贝尔编程！")
            break
        if name != username:
            print("用户名错误！请重新输入！"+"还剩"+str(i)+"次机会可输入！")
            continue
        if password != userpassword:    
            print("密码错误!请重新输入"+"还剩"+str(i)+"次机会可输入！")
    else:   
        print("三次输入机会已用完！请再单击一次屏幕上方的“运行”按钮！")
        exit()