class Foo():
    def __init__(self, a):
        self.a = a

    def forward(self, x):
        x = self.__apply(x)
        return x * self.a

    def __apply(self, x):
        x = x * 2
        return x

class Bar(Foo):
    def __apply(self, x):
        x = x * (-2)
        return x

clas = Bar(1)
clas.forward(1)  # вернется 2, а я хочу -2