This method is called when the flow is completed.

def finalize(self):
        """This method is called when the flow is completed."""
        # Merge all the out_DDB files found in work.outdir.
        ddb_files = list(filter(None, [work.outdir.has_abiext("DDB") for work in self]))

        # Final DDB file will be produced in the outdir of the work.
        out_ddb = self.outdir.path_in("out_DDB")
        desc = "DDB file merged by %s on %s" % (self.__class__.__name__, time.asctime())

        mrgddb = wrappers.Mrgddb(manager=self.manager, verbose=0)
        mrgddb.merge(self.outdir.path, ddb_files, out_ddb=out_ddb, description=desc)
        print("Final DDB file available at %s" % out_ddb)

        # Call the method of the super class.
        retcode = super().finalize()
        return retcode