class a(object):
    __numbers = [1, 2, 3]

    def input(self, numbers):
        self.__numbers = numbers

    def sum(self):
        print(sum(self.__numbers))


class b(a):
   def sum_x2(self):
       print(2 * sum(self.__numbers))  # this will NOT work

   def sum_x2_renamed(self):
       print(2 * sum(self._a__numbers))  # this will work fine


my_object = b()
my_object.input([1, 5, 10])
my_object.sum_x2_renamed()