text: # Define here the models for your spider middleware
  #
  # See documentation in:
  # https://docs.scrapy.org/en/latest/topics/spider-middleware.html
  
  from scrapy import signals
  
  # useful for handling different item types with a single interface
  from itemadapter import is_item, ItemAdapter
  
  '''
  此类常用来写爬虫中间件: 一般有DOWNLOADER_MIDDLEWARES（下载中间件）和SPIDER_MIDDLEWARES（爬虫中间件），下载中间件最常用
  Downloader Middlewares默认的方法有：
      process_request(self, request, spider):
          1.当每个request通过下载中间件时，该方法被调用
          2.返回None值：没有returen也是返回None，该request对象传递給下载器，或通过引擎传递给其他权重低的process_request方法
          3.返回Response对象：不在请求，把response返回给引擎
          4.返回Request对象：把request对象通过引擎交给调度器，此时将不在通过其他权重低的process_request方法
      process_response(self, response, spider):
          1.当下载器完成http请求，传递响应给引擎的时候嗲用
          2.返回Response：通过引擎交给爬虫处理或者交给权重更低的其他下载中间件的process_response方法
          3.返回Request对象：通过引擎交给调度器继续请求，此时将不通过其他权重低的process_request方法
      在settings.py中开启中间件，权重越小越优先执行
  
  '''
  
  class ScrapyTaobaoSpiderMiddleware:
      # Not all methods need to be defined. If a method is not defined,
      # scrapy acts as if the spider middleware does not modify the
      # passed objects.
  
      @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_spider_input(self, response, spider):
          # Called for each response that goes through the spider
          # middleware and into the spider.
  
          # Should return None or raise an exception.
          return None
  
      def process_spider_output(self, response, result, spider):
          # Called with the results returned from the Spider, after
          # it has processed the response.
  
          # Must return an iterable of Request, or item objects.
          for i in result:
              yield i
  
      def process_spider_exception(self, response, exception, spider):
          # Called when a spider or process_spider_input() method
          # (from other spider middleware) raises an exception.
  
          # Should return either None or an iterable of Request or item objects.
          pass
  
      def process_start_requests(self, start_requests, spider):
          # Called with the start requests of the spider, and works
          # similarly to the process_spider_output() method, except
          # that it doesn’t have a response associated.
  
          # Must return only requests (not items).
          for r in start_requests:
              yield r
  
      def spider_opened(self, spider):
          spider.logger.info('Spider opened: %s' % spider.name)
  
  
  class ScrapyTaobaoDownloaderMiddleware:
      # Not all methods need to be defined. If a method is not defined,
      # scrapy acts as if the downloader middleware does not modify the
      # passed objects.
  
      @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):
          # 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
          return None
  
      def process_response(self, request, response, spider):
          # Called with the response returned from the downloader.
  
          # Must either;
          # - return a Response object
          # - return a Request object
          # - or raise IgnoreRequest
          return response
  
      def process_exception(self, request, exception, spider):
          # Called when a download handler or a process_request()
          # (from other downloader middleware) raises an exception.
  
          # Must either:
          # - return None: continue processing this exception
          # - return a Response object: stops process_exception() chain
          # - return a Request object: stops process_exception() chain
          pass
  
      def spider_opened(self, spider):
          spider.logger.info('Spider opened: %s' % spider.name)
  