在字典上使用@property装饰器 [英] Using @property decorator on dicts

问题描述

我正在尝试在类中的字典上使用Python的@property装饰器.我的想法是我希望在访问某个值后将其清除(称为消息").但是我还希望另一个值(称为"last_message")包含最后设置的消息，并将其保留直到设置了另一个消息.在我看来，这段代码可以正常工作:

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"]
...
>>>

但是，似乎没有:

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

我不确定我做错了什么?在我看来，@property不能像我期望的那样工作，但是也许我在做其他根本上错误的事情?

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?

此外，我知道我可以在类中使用单个值.但这是作为Web应用程序中的会话实现的，我需要将它作为命令.我可以做这项工作，也可以使整个会话对象假装成字典，或者使用单个变量将其遍历整个代码库的其余部分.我宁愿让它开始工作.

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':
            super().__setitem__('message', '')
            super().__setitem__('last_message', value) 
        else:
            super().__setitem__(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

正如我认为您已经发现的那样，您的二传手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'

首先访问a.b，它调用b属性的getter，而不是其setter.吸气剂返回字典self._b.然后self._b['message']='hello'导致字典的__setitem__被称为.

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 .

因此，要解决此问题，您需要一个特殊的字典(例如MyDict).

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

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