code: def _update_cache_stats(self, key, result):
          
          if result is None:
              self._CACHE_STATS['access_stats'].setdefault(key,
                                           {'hit': 0, 'miss': 0, 'expired': 0})
          else:
              self._CACHE_STATS['access_stats'][key][result] +=1
docs: Update the cache stats.
          
          If no cache-result is specified, we iniitialize the key.
          Otherwise, we increment the correct cache-result.
  
          Note the behavior for expired.  A client can be expired and the key
          still exists.