def _shipping_sub_total(self):
        if self.shipping_cost is None:
            self.shipping_cost = Decimal('0.00')
        if self.shipping_discount is None:
            self.shipping_discount = Decimal('0.00')
        return self.shipping_discount-self.shipping_cost
dataset/ETHPy150Open dokterbob/satchmo/satchmo/apps/satchmo_store/shop/models.py/Order._shipping_sub_total/swapped operands of `-`