初识python yield

for sel in response.xpath('//ul/li'):
    item = DmozItem()
    item['title'] = sel.xpath('a/text()').extract()
    item['link'] = sel.xpath('a/@href').extract()
    item['desc'] = sel.xpath('text()').extract()
    yield item

等价于：

items []
for sel in response.xpath('//ul/li'):
    item = DmozItem()
    item['title'] = sel.xpath('a/text()').extract()
    item['link'] = sel.xpath('a/@href').extract()
    item['desc'] = sel.xpath('text()').extract()
　　 items.append(item)
    return items

就这么简单！