def METHOD_NAME(self):
    partner2 = self.env["res.partner"].create(
        {"name": "partner 2", "delivery_zone_id": self.delivery_zone_b.id}
    )
    self.order.action_confirm()
    self.order.picking_ids.partner_id = partner2
    self.assertEqual(self.order.picking_ids.delivery_zone_id, self.delivery_zone_b)