yield优点
yield 的好处是显而易见的,把一个函数改写为一个 generator 就获得了迭代能力,比起用类的实例保存状态来计算下一个 next() 的值,不仅代码简洁,而且执行流程异常清晰
而在Scrapy爬虫框架中,yield有天然的使用场景,因为我们并不知道爬虫每次获取数据的大小,如果每次都一起返回则数据量会非常大,此时如果采用yield来优化,则代码会非常简洁且高效
yield优化返回数据
# -*- coding: utf-8 -*- import scrapy from lxml import etree from douban.items import DoubanItem class DoubanSpiderSpider(scrapy.Spider): name = 'douban_spider' # 允许下载的域名 allowed_domains = ['movie.douban.com'] # 配置下载的首地址 start_urls = ['http://movie.douban.com/top250'] # 下载完毕之后的解析方法 (parse在源码中支持yield) def parse(self, response): # print(response.text) html = etree.HTML(response.text) # 首先通过xpath获取ol li_list = html.xpath("//ol[@class='grid_view']/li") for li in li_list: item = DoubanItem() # em = title = img = comment item['em'] = li.xpath(".//em/text()")[0] item['title'] = li.xpath(".//span[@class='title']/text()")[0] item['img'] = li.xpath(".//img/@src")[0] item['comment'] = li.xpath(".//div[@class='star']/span/text()")[-1] # yield返回当前电影的数据 yield item try: # 获取后页超链接的值 (xpath返回的是list) next_page = html.xpath("//span[@class='next']/a/@href")[0] # 手动发送请求,让爬虫去解析下一页的数据 (ajax) yield scrapy.Request(url = 'http://movie.douban.com/top250' + next_page,callback=self.parse) except: print('下载完毕.......')