这个问题在这里已经有答案了：

不退不退(5个答案）
一小时前关门了。
当你拿一本书的例子来胡闹的时候：

class Quick():
    def __str__(self):
        return "\n".join(f"{k}: {v}" for k, v in vars(self).items())

class User(Quick):

    def __init__(self, first, last, location, gender):
        super().__init__()
        self.first = first
        self.last = last
        self.location = location
        self.gender = gender
        self.loginattempt = 0

    def mmplusattempt(self):
        self.loginattempt += 1

    def mmresetattempt(self):
        self.loginattempt = 0

    def __str__(self):
        return "\n".join(f"{k}: {v}" for k, v in vars(self).items())

class Admin(User):

    def __init__(self, first, last, location, gender):
        super().__init__(first, last, location, gender)
        self.privileges = ["delete post", "ban user", "shutdown server", "restart server"]

    def show(self):
        for p in self.privileges:
            print(f"This user can: {p}. \n")

admin = Admin("ADMIN", "ADMIN", "SERVER ROOM", "ADMIN")
print(admin.show())

出于某种原因，这会输出：

This user can: delete post. 

This user can: ban user. 

This user can: shutdown server. 

This user can: restart server. 

None

我不知道为什么会打印出来 None . 改变 show() 要检查空值：

def show(self):
        for p in self.privileges:
            if p != None:
                print(f"This user can: {p}. \n")

静止输出 None .
我想可能循环没有像我预期的那样工作，所以在中添加了一个空值 self.privileges 要查看循环是否正确检查空值，请执行以下操作： self.privileges = ["delete post", "ban user", "shutdown server", None, "restart server"] 输出：

└─$ python3 classuser.py
This user can: delete post. 

This user can: ban user. 

This user can: shutdown server. 

This user can: restart server. 

None

显然，if测试工作正常。这个空值从何而来？