code: def GetEventTagByIdentifier(self, storage_file, event_identifier):
      
      if not self._index:
        self._Build(storage_file)
  
      lookup_key = event_identifier.CopyToString()
      event_tag_identifier = self._index.get(lookup_key, None)
      if not event_tag_identifier:
        return None
  
      return storage_file.GetEventTagByIdentifier(event_tag_identifier)
docs: Retrieves the most recently updated event tag for an event.
  
      Args:
        storage_file (BaseStorageFile): storage file.
        event_identifier (AttributeContainerIdentifier): event attribute
            container identifier.
  
      Returns:
        EventTag: event tag or None if the event has no event tag.