python函数在打印列表时打印空值?

dced5bon  于 2021-07-13  发布在  Java
关注(0)|答案(1)|浏览(372)

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?

brtdzjyr

brtdzjyr1#

show 如果在函数中返回某个值,则函数没有返回任何值 show , None 我要走了。

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

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