scrapy的中间件（下载中间件）

# 下载中间件
-process_request：返回不同的对象，后续处理不同（加代理...）


class CnblogsDownloaderMiddleware:
    @classmethod
    def from_crawler(cls, crawler):
        pass
    def process_request(self, request, spider):
        # Called for each request that goes through the downloader
        # middleware.

        # Must either:
        # - return None: continue processing this request
        # - or return a Response object
        # - or return a Request object
        # - or raise IgnoreRequest: process_exception() methods of
        #   installed downloader middleware will be called

        # 1 更换请求头from scrapy.http.headers import Headers
        # 方式一：
        # request.headers['User-Agent'] = 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.105 Safari/537.36'

        # 方式二：使用fake-useragent
        # pip3 install fake-useragent
        # from fake_useragent import UserAgent
        # request.headers['User-Agent'] = UserAgent().random
        # print(request.headers)

        # 2 加cookie ---cookie池
        # 假设你你已经搭建好cookie 池了，
        # print('00000--',request.cookies)
        # request.cookies={'username':'asdfasdf'}

        # 3 加代理
        # print(request.meta)
        # request.meta['download_timeout'] = 20
        # request.meta["proxy"] = 'http://218.22.7.62:53281'