Descriptor example using weak key dictionary

import weakref

class Foo:
    def __init__(self):
        self._data = weakref.WeakKeyDictionary()
    def __get__(self, obj, owner):
        if obj is None:
            return self
        result = self._data.get(obj, None)
        # do something to result
        return result
    def __set__(self, obj, value):
        # do something to value
        self._data[obj] = value
    def __delete__(self, obj):
        del self._data[obj]

class Bar:
    foo = Foo()