登录成功就break，登录不成功就打印

_user = "xiaoyanzi"_passwd = "woaini"pass_authentication = Falsefor i in range(3):    username = input("username:")    passwd = input("passwd:")    if _user == username and _passwd == passwd:        print("welcome to %s login"%username)        pass_authentication = True        break    else:        print("invalid username or passwd !!!")if not pass_authentication:    print("不洗脚的小燕子！！！")

可精简为：

_user = "xiaoyanzi"_passwd = "woaini"for i in range(3):    username = input("username:")    passwd = input("passwd:")    if _user == username and _passwd == passwd:        print("welcome to %s login"%username)        break    else:        print("invalid username or passwd !!!")else:    print("不洗脚的小燕子！！！")

while循环（无限循环，又称死循环）：

while true:print("中国人民万岁！")

输出结果为n个：

中国人民万岁！

_user = "xiaoyanzi"_passwd = "woaini"while true:  #死循环    username = input("username:")    passwd = input("passwd:")    if _user == username and _passwd == passwd:        print("welcome to %s login"%username)        break    else:        print("invalid username or passwd !!!")else:    print("不洗脚的小燕子！！！")

将死循环改为有限循环：

_user = "xiaoyanzi"_passwd = "woaini"counter = 0while counter < 3:    username = input("username:")    passwd = input("passwd:")    if _user == username and _passwd == passwd:        print("welcome to %s login"%username)                break    else:        print("invalid username or passwd !!!")    counter += 1else:    print("不洗脚的小燕子！！！")

三次输入错误之后，是否还想继续3次，再继续3次，效果：

_user = "xiaoyanzi"_passwd = "woaini"counter = 0while counter < 3:    username = input("username:")    passwd = input("passwd:")    if _user == username and _passwd == passwd:        print("welcome to %s login"%username)        True        break    else:        print("invalid username or passwd !!!")    counter += 1        if counter == 3:        keep_going_choice = input ("还想玩么？[y/n]")        if keep_going_choice == "y":            counter = 0        else:    print("不洗脚的小燕子！！！")