如何在python中创建无法收集的垃圾？

class Uncollectable:
    _instances = []

    def __init__(self, value):
        self.value = value
        self._instances.append(self)

    def __del__(self):
        self._instances.remove(self)

class Uncollectable:
    _instances = []

    def __init__(self, value):
        self.value = value
        self._instances.append(self)
        self.__weakref__ = weakref.ref(self)

    def __del__(self):
        self._instances.remove(self)

obj = Uncollectable(42)