code: def output_file(self):
          
          out_files = self.output_files
          if len(out_files) != 1:
              err_msg = "output_file property is only valid if there is a single"
              err_msg += " output file. Here there are "
              err_msg += "%d output files." %(len(out_files))
              raise ValueError(err_msg)
          return out_files[0]
docs: If only one output file return it. Otherwise raise an exception.