scrpy回调问题

Question

def parse(self, response):
   post_nodes = response.css('.search_txt h3 a')

   for post_url in post_nodes.css("::attr(href)").extract_first(""):
       post_url=parse.urljoin(response.url, post_url)
       yield Request(url=post_url,callback=self.parse_item)

我这段代码为啥回调不到parse_item