程序示例：

users = {"user1": "123456", "user2": "123456", "user3": "123456"}
blacklist = ["user4", "user5", "user6"]
count = 1
while True:
    name = input("Enter your name (q to quit): ")
    if name == 'q':
        break
    else:
        # 用戶名存在且不在黑名單內
        if name in users.keys():
            for i in range(3):
                password = input("Enter your password:")
                count += 1  # 每輸入一次密碼，計數器加 1
                if password == users[name]:
                    print("Welcome " + name + "!")
                    break
                else:
                    if count < 4:
                        print("Sorry " + name + "'s password is incorrect. Enter a different password: ")
            # 輸出三次密碼都不對，count 初始值為 1，此時為 4
            if count == 4:
                print("Sorry " + name + "'s password is incorrect. You have no chance anymore!")
        # 在黑名單里
        elif name in blacklist:
            print("Sorry " + name + ", you are in blacklist!")
        # 用戶名不存在，可以注冊
        else:
            print("Sorry " + name + ", you are not registered! You can register now!")
            password = input("Enter your password:")
            print("Welcome " + name + "! You are now logged on!")
            users[name] = password