要求：使用者正確輸入使用者名稱和密碼便成功登陸，分別有三次機會輸入使用者名稱和密碼，超過3次便鎖定

分析：用兩個while迴圈即可，程式碼如下：

user_name = "Brett"
password = "1314"

i = 0
n = 0
Is_exit = False  #進入迴圈標誌
while not Is_exit:
    User_name = input("please input your username:")
    if  User_name == user_name:
        while not Is_exit:
            Password = input("please input your password:")
            if Password == password:
                print("welcome!")
                Is_exit = True
                break
            elif n >= 2:
                print("your times are used out!")
                Is_exit = True
                break
            else:
                print("your password is not true, please try again!")
                n = n+1
    elif i >= 2:
        print("your times are used out!")
        Is_exit = True
        break
    else:print("your username is not true, please try again!")
    i = i + 1