code: def _braket_fmt(self, expr_type):
          
          mapping = {
              'bra': {
                  True: '<{label}|^({space})',
                  'subscript': '<{label}|_({space})',
                  False:  '<{label}|'},
              'ket': {
                  True: '|{label}>^({space})',
                  'subscript': '|{label}>_({space})',
                  False:  '|{label}>'},
              'ketbra': {
                  True: '|{label_i}><{label_j}|^({space})',
                  'subscript': '|{label_i}><{label_j}|_({space})',
                  False:  '|{label_i}><{label_j}|'},
              'braket': {
                  True: '<{label_i}|{label_j}>^({space})',
                  'subscript': '<{label_i}|{label_j}>_({space})',
                  False:  '<{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