code: def generate_key(action, path_or_id, settings=None, default=" (default)"):
      
      settings = " {}".format(str(sorted(settings.items()))) if settings else default
      return "{}: {}{}".format(action.upper(), path_or_id, settings)
docs: generate_key: generate key used for caching
          Args:
              action (str): how video is being processed (e.g. COMPRESSED or DOWNLOADED)
              path_or_id (str): path to video or youtube_id
              settings (dict): settings for compression or downloading passed in by user
              default (str): if settings are None, default to this extension (avoid overwriting keys)
          Returns: filename