如何返回一个子类独占的变量？

class ParentClass:
    def __init__(self):
        self.__private_variable = None

    def get_private_variable(self):
        return self.__private_variable


class ChildClass(ParentClass):
    def set_private_variable(self, value):
        self._ParentClass__private_variable = value

    def get_private_variable(self):
        return "ChildClass: " + str(self._ParentClass__private_variable)


parent = ParentClass()
child = ChildClass()

parent.set_private_variable(10)
child.set_private_variable(20)

print(parent.get_private_variable())  # 输出：10
print(child.get_private_variable())  # 输出：ChildClass: 20