code: def save_to_metadata_file(cls, dirpath, key, value):
          
          fullpath = os.path.join(dirpath, cls.metadata_filename)
          if os.path.exists(fullpath):
              d = pickle.load(open(fullpath, 'rb'))
              d.update({key: value})
              pickle.dump(d, open(fullpath, 'wb'))
          else:
              pickle.dump({key: value}, open(fullpath, 'wb'))
docs: Store `key: value` in metadata file dict.