docs: def _format_output(self, outfile_name, out_type):
          """ Prepend proper output prefix to output filename """
  
          outfile_name = self._absolute(outfile_name)
          outparts = outfile_name.split("/")
          outparts[-1] = self._out_format % (out_type, outparts[-1] )
  
          return '/'.join(outparts)