class PromoService():

    @classmethod
    def create_promo(cls, promotion):
        #do stuff
        return promo

class DiscountPromoService():

    @classmethod
    def create_promo(cls, promo, discount):
        promo = super(DiscountPromoService, cls).create_promo(promo)
        promo.discount = discount
        promo.save()
        return promo