A custom dict (second try)

def set_of_attrs(cls):
    cls._attrs = frozenset(dir(cls))
    return cls

@set_of_attrs
class pdict(dict):
    @property
    def foo(self):
        print("foo.getter")

    @foo.setter
    def foo(self, value):
        print("foo.setter", value)

    def __getattr__(self, name):
        return self.get(name, None)

    def __setattr__(self, name, value):
        print("In pdict.__setattr__(self, %r, %r):" % (name, value))
        if name in self._attrs:
            print("\tsuper().__setattr__(%r, %r)" % (name, value))
            super().__setattr__(name, value)
        else:
            print("\tself[%r] = %r" % (name, value))
            self[name] = value