def _Build(self, storage_file):
    """Builds the event tag index.

    Args:
      storage_file (BaseStorageFile): storage file.
    """
    self._index = {}
    for event_tag in storage_file.GetEventTags():
      self.SetEventTag(event_tag)
Builds the event tag index.

    Args:
      storage_file (BaseStorageFile): storage file.