code: def feed(self, url_template, keyword, offset, max_num, page_step):
          
          for i in range(offset, offset + max_num, page_step):
              url = url_template.format(keyword, i)
              self.out_queue.put(url)
              self.logger.debug('put url to url_queue: {}'.format(url))
docs: Feed urls once
  
          Args:
              url_template: A string with parameters replaced with "{}".
              keyword: A string indicating the searching keyword.
              offset: An integer indicating the starting index.
              max_num: An integer indicating the max number of images to be crawled.
              page_step: An integer added to offset after each iteration.