用户登录系统

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:
        # 用户名存在且不在黑名单内
        if name in users.keys():
            for i in range(3):
                password = input("Enter your password:")
                count += 1  # 每输入一次密码，计数器加 1
                if password == users[name]:
                    print("Welcome " + name + "!")
                    break
                else:
                    if count < 4:
                        print("Sorry " + name + "'s password is incorrect. Enter a different password: ")
            # 输出三次密码都不对，count 初始值为 1，此时为 4
            if count == 4:
                print("Sorry " + name + "'s password is incorrect. You have no chance anymore!")
        # 在黑名单里
        elif name in blacklist:
            print("Sorry " + name + ", you are in blacklist!")
        # 用户名不存在，可以注册
        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

另一种写法：

程序示例：

users = {"user1": {"password": 12345, "status": True}, "user2": {"password": 12345, "status": True},
         "user3": {"password": 12345, "status": False}}
for i in range(3):
    name = input("Enter your name: ")
    password = int(input("Enter your password: "))
    # 用户名存在且不在黑名单内，且用户名输入正确
    if name in users.keys() and users[name]["password"] == password and users[name]['status']:
        print("Welcome " + name + "!")
        break
    # 用户名存在且在黑名单内
    elif name in users.keys() and not users[name]['status']:
        print("Sorry " + name + ", you are prohibited from logging in!")
        break
    elif name in users.keys() and password != users[name]['password']:
        print("Sorry " + name + ", your password does not match!")
    elif name not in users.keys():
        print("Sorry " + name + ", you are not logged in!")
        break