code: def _braket_fmt(self, expr_type):
          
          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]
docs: Return a format string for printing an `expr_type`
          ket/bra/ketbra/braket