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"]
...
>>>
However, it doesn't seem to:
>>> a = A()
>>> a.b["message"] = "hello"
>>> a.b["message"]
''
>>> a.b["last_message"]
''
>>>
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?
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.
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
).
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
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