middlewares.py

class MiddlewareDownloaderMiddleware:

    @classmethod
    def from_crawler(cls, crawler):
        # This method is used by Scrapy to create your spiders.
        s = cls()
        crawler.signals.connect(s.spider_opened, signal=signals.spider_opened)
        return s

    def process_request(self, request, spider):
        # spider就是爬虫类的实例化对象
        # spider.name
        # 拦截所有的请求对象，包括正常与不正常
        # 参数：request就是请求到的对象
        # 获取或者修改请求头信息
        # request.headers['Cookie'] = 'xxx'
        print('i am process_request ')
        return None

    def process_response(self, request, response, spider):
        # 拦截所有的响应对象
        # 参数：response就是响应对象
        print('i am process_response ')
        return response

    def process_exception(self, request, exception, spider):
        # 拦截发生异常的请求对象
        # 需要对异常的请求进行修正，然后将其重新发送即可
        print('i am process_exception ')
        # 代理操作
        # request.meta['proxy'] = 'https://ip:port'
        return request

settings.py 开启中间件

DOWNLOADER_MIDDLEWARES = {
   'middleware.middlewares.MiddlewareDownloaderMiddleware': 543,
}