code: def _index(self):
          
          if self.__index is None:
              try:
                  with open(self._get_path('index.json')) as f:
                      data = json.load(f)
              except (IOError, ValueError):
                  self.__index = {}
              else:
                  # 0 means version is not defined (= always delete cache):
                  if data.get('version', 0) != self.version:
                      self.clear_cache()
                      self.__index = {}
                  else:
                      self.__index = data['index']
          return self.__index
docs: Keys a list of file paths that have been pickled in this directory.
          The index is stored in a json file in the same directory as the 
          pickled objects.