code: def set_fetcher(self, fetcher_class):
          
          self._lock.acquire()
          try:
              self._fetcher = fetcher_class
          finally:
              self._lock.release()
docs: Set the fetcher class.
  
          :Parameters:
              - `fetcher_class`: the fetcher class.
          :Types:
              - `fetcher_class`: `CacheFetcher` based class