code: def get_photo_url(photo_id):
      
      args = _get_request_args(
          'flickr.photos.getSizes',
          photo_id=photo_id
      )
      resp = requests.post(API_URL, data=args)
      resp_json = json.loads(resp.text.encode('utf-8'))
      logger.debug(json.dumps(resp_json, indent=2))
      size_list = resp_json['sizes']['size']
      size_list_len = len(size_list)
      global image_size_mode
      image_size_mode = size_list_len if size_list_len < image_size_mode \
          else image_size_mode
      download_url = resp_json['sizes']['size'][-image_size_mode]['source']
      return download_url
docs: Request the photo download url with the photo id
      :param photo_id: The photo id of flickr
      :type photo_id: str
      :return: Photo download url
      :rtype: str