在Python上使用Python的@property装饰器 [英] Using Python's @property decorator on dicts

问题描述

我正在尝试在类中的dict上使用Python的 @property 装饰器。这个想法是，我想要一个值（被称为'message'）在被访问之后被清除。但是我也想要另外一个值（称为'last_message'）来包含最后一个设置的消息，并保留它，直到设置另一个消息。在我看来，这段代码可以工作：

I'm trying to use Python's @property decorator on a dict in a class. The idea is that I want a certain value (call it 'message') to be cleared after it is accessed. But I also want another value (call it 'last_message') to contain the last set message, and keep it until another message is set. In my mind, this code would work:

>>> class A(object):
...     def __init__(self):
...             self._b = {"message": "", 
...                        "last_message": ""}
...     @property
...     def b(self):
...             b = self._b
...             self._b["message"] = ""
...             return b
...     @b.setter
...     def b(self, value):
...             self._b = value
...             self._b["last_message"] = value["message"]
...
>>>

然而，似乎没有：

>>> a = A()
>>> a.b["message"] = "hello"
>>> a.b["message"]
''
>>> a.b["last_message"]
''
>>>

我不知道我做错了什么？在我看来，像 @property 不像我期望的那样对dicts有效，但也许我正在做一些其他从根本上错误的事情？

I'm not sure what I have done wrong? It seems to me like @property doesn't work like I would expect it to on dicts, but maybe I'm doing something else fundamentally wrong?

另外，我知道我可以在类中使用单个值。但是这是在Web应用程序中实现的一个会话，我需要它作为一个dict。我可以做这个工作，或者使整个session对象假装它是一个dict，或者使用单个的变量，并把它整个代码基础的其余部分进行工作。我很愿意让这个工作。

Also, I know that I could just use individual values in the class. But this is implemented as a session in a web application and I need it to be a dict. I could either make this work, or make the whole session object to pretend it's a dict, or use individual variables and hack it into workingness throughout the rest of the code base. I would much rather just get this to work.

推荐答案

class MyDict(dict):
    def __setitem__(self,key,value):
        if key=='message':
            dict.__setitem__(self,'message','')
            dict.__setitem__(self,'last_message',value) 
        else:
            dict.__setitem__(self,key,value)

class A(object):
    def __init__(self):
            self._b = MyDict({"message": "", 
                       "last_message": ""})
    @property
    def b(self):
        return self._b

a=A()
a.b['message']='hello'
print(a.b['message'])
# ''
print(a.b['last_message'])
# hello

我想你已经发现，你的设置者不工作的原因是因为

As I think you've discovered, the reason why your setter wasn't working is because

a.b['message']='hello'

首次访问 a.b ，它调用了 b 属性的getter，而不是其setter。 getter返回dict self._b 。然后 self._b ['message'] ='hello'导致dict的 __ setitem __ 被调用。

first accesses a.b, which calls the b property's getter, not its setter. The getter returns the dict self._b. Then self._b['message']='hello' causes the dict's __setitem__ is called .

所以要解决这个问题，你需要一个特殊的dict（如 MyDict ）。

So to fix the problem, you need a special dict (like MyDict).

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