Return a format string for printing an `expr_type`
        ket/bra/ketbra/braket

def _braket_fmt(self, expr_type):
        """Return a format string for printing an `expr_type`
        ket/bra/ketbra/braket"""
        if self._settings['unicode_sub_super']:
            sub_sup_fmt = SubSupFmt
        else:
            sub_sup_fmt = SubSupFmtNoUni
        mapping = {
            'bra': {
                True: sub_sup_fmt('⟨{label}|', sup='({space})'),
                'subscript': sub_sup_fmt('⟨{label}|', sub='({space})'),
                False:  sub_sup_fmt('⟨{label}|')},
            'ket': {
                True: sub_sup_fmt('|{label}⟩', sup='({space})'),
                'subscript': sub_sup_fmt('|{label}⟩', sub='({space})'),
                False:  sub_sup_fmt('|{label}⟩')},
            'ketbra': {
                True: sub_sup_fmt('|{label_i}⟩⟨{label_j}|', sup='({space})'),
                'subscript': sub_sup_fmt(
                    '|{label_i}⟩⟨{label_j}|', sub='({space})'),
                False:  sub_sup_fmt('|{label_i}⟩⟨{label_j}|')},
            'braket': {
                True: sub_sup_fmt('⟨{label_i}|{label_j}⟩', sup='({space})'),
                'subscript': sub_sup_fmt(
                    '⟨{label_i}|{label_j}⟩', sub='({space})'),
                False:  sub_sup_fmt('⟨{label_i}|{label_j}⟩')},
        }
        hs_setting = bool(self._settings['show_hs_label'])
        if self._settings['show_hs_label'] == 'subscript':
            hs_setting = 'subscript'
        return mapping[expr_type][hs_setting]