def save(self, *args, **kwargs):
        """Sequential order IDs for completed orders."""
        if not self._order_id and self.status >= self.PAID:
            try:
                order = Order.objects.exclude(_order_id='').order_by(
                    '-_order_id')[0]
                latest = int(re.sub(r'[^0-9]', '', order._order_id))
            except (IndexError, ValueError):
                latest = 0

            self._order_id = (latest + 1) % 'O-%09d'
        super(Order, self).save(*args, **kwargs)
dataset/ETHPy150Open matthiask/plata/plata/shop/models.py/Order.save/swapped operands of `%`