code: def _FormatSource(self, event):
      
      _, source = self._output_mediator.GetFormattedSources(event)
      if source is None:
        data_type = getattr(event, 'data_type', 'UNKNOWN')
        raise errors.NoFormatterFound(
            'Unable to find event formatter for: {0:s}.'.format(data_type))
  
      return source
docs: Formats the source.
  
      Args:
        event (EventObject): event.
  
      Returns:
        str: source field.
  
      Raises:
        NoFormatterFound: if no event formatter can be found to match the data
            type in the event.