（Python）防止在静态类中动态添加变量

Question

我一直在使用以下方法来“冻结”Python中的类实例，这样如果我无意中定义了一个新的实例变量，就会出现错误，而不仅仅是让我这样做。

class FrozenClass(object):
        __isfrozen = False

        def __setattr__(self, key, value):
            if self.__isfrozen and key not in dir(self):
                raise TypeError( "%r is a frozen class" % self )

            object.__setattr__(self, key, value)

        def _freeze(self):
            self.__isfrozen = True

        ## so that if you're inheriting a frozen class you can unfreeze it 
        def _unfreeze(self):
            self.__isfrozen = False

其中有效：

>>> class Foo(FrozenClass):
...     def __init__(self):
...             self.a = 1
...             self._freeze()
...
>>> myInstance = Foo()
>>> myInstance.A = 4
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/XXX/Desktop/Git/functions/FrozenClass.py", line 6, in __setattr__
    raise TypeError( "%r is a frozen class" % self )
TypeError: <__main__.Foo object at 0x10075c4e0> is a frozen class
>>>

然而，我一直在使用一个全局数据对象，我已经使其成为静态的，但它不起作用。我试过玩它但无济于事。这就是现在的情况：

## doesn't currently work. Not sure. 
class FrozenStaticClass(object):
    __isfrozen = False

    def __setattr__(cls, key, value):
        if cls.__isfrozen and key not in dir(cls):
            raise TypeError( "%r is a frozen class" % cls )

        object.__setattr__(cls, key, value)

    @classmethod
    def _freeze(cls):
        cls.__isfrozen = True

    @classmethod
    def _unfreeze(cls):
        cls.__isfrozen = False

如何让它冻结静态类？

编辑： @volcano建议使用插槽但它似乎不适用于类变量。