Using @property decorator on dicts

Question

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 a

Answer 1

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

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

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

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 .

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

Answer 2

I may be missing what you are trying to do here, but does this solve your problem?

class A(object):
    def __init__(self):
        self._b = {'message':'',
                   'last_message': ''}

    @property
    def b(self):
        b = self._b.copy()
        self._b['message'] = ''
        return b

    @b.setter
    def b(self, value):
        self._b['message'] = value
        self._b['last_message'] = value


if __name__ == "__main__":
    a = A()
    a.b = "hello"
    print a.b
    print a.b
    print a.b["last_message"]

---

$ python dictPropTest.py
{'last_message': 'hello', 'message': 'hello'}
{'last_message': 'hello', 'message': ''}
hello