code: def _cache_index(self, dbname, collection, index, cache_for):
          
          now = datetime.datetime.utcnow()
          expire = datetime.timedelta(seconds=cache_for) + now
  
          with self.__index_cache_lock:
              if dbname not in self.__index_cache:
                  self.__index_cache[dbname] = {}
                  self.__index_cache[dbname][collection] = {}
                  self.__index_cache[dbname][collection][index] = expire
  
              elif collection not in self.__index_cache[dbname]:
                  self.__index_cache[dbname][collection] = {}
                  self.__index_cache[dbname][collection][index] = expire
  
              else:
                  self.__index_cache[dbname][collection][index] = expire
docs: Add an index to the index cache for ensure_index operations.