def _FormatSource(self, event):
    """Formats the source.

    Args:
      event (EventObject): event.

     Returns:
       str: formatted source field.
    """
    source_short, _ = self._output_mediator.GetFormattedSources(event)
    if source_short is None:
      data_type = getattr(event, 'data_type', 'UNKNOWN')
      raise errors.NoFormatterFound(
          'Unable to find event formatter for: {0:s}.'.format(data_type))

    return self._SanitizeField(source_short)
Formats the source.

    Args:
      event (EventObject): event.

     Returns:
       str: formatted source field.