def _write_nmlgrp(self, grp_name, grp_vars, nml_file, sort=False):
        """Write namelist group to target file."""
        if self._newline:
            print(file=nml_file)
        self._newline = True

        if self.uppercase:
            grp_name = grp_name.upper()

        if sort:
            grp_vars = Namelist(sorted(grp_vars.items(), key=lambda t: t[0]))

        print('&{0}'.format(grp_name), file=nml_file)

        for v_name, v_val in grp_vars.items():

            v_start = grp_vars.start_index.get(v_name, None)

            for v_str in self._var_strings(v_name, v_val, v_start=v_start):
                nml_line = self.indent + '{0}'.format(v_str)
                print(nml_line, file=nml_file)

        print('/', file=nml_file)
Write namelist group to target file.