def set_group(self, group):
        """Set group size of Dataset (used for ranking).

        Parameters
        ----------
        group : list, numpy 1-D array, pandas Series or None
            Group size of each group.

        Returns
        -------
        self : Dataset
            Dataset with set group.
        """
        self.group = group
        if self.handle is not None and group is not None:
            group = list_to_1d_numpy(group, np.int32, name='group')
            self.set_field('group', group)
        return self