Parameters
        ----------
        obj : the subject object
        validate : boolean, default True
            if True, validate the grouper

        Returns
        -------
        a tuple of binner, grouper, obj (possibly sorted)

def _get_grouper(self, obj, validate=True):
        """
        Parameters
        ----------
        obj : the subject object
        validate : boolean, default True
            if True, validate the grouper

        Returns
        -------
        a tuple of binner, grouper, obj (possibly sorted)
        """

        self._set_grouper(obj)
        self.grouper, exclusions, self.obj = _get_grouper(self.obj, [self.key],
                                                          axis=self.axis,
                                                          level=self.level,
                                                          sort=self.sort,
                                                          validate=validate)
        return self.binner, self.grouper, self.obj