code: def return_attr(self, attr, labels=None):
          
          all_labels = self.return_label()
  
          if labels is None:
              labels = all_labels
  
          all_attr = []
          for one_label in labels:
              idx = all_labels.index(one_label)
              try:
                  all_attr.append(self.chan[idx].attr[attr])
              except KeyError:
                  possible_attr = ', '.join(self.chan[idx].attr.keys())
                  lg.debug('key "{}" not found, '.format(attr) +
                           'possible keys are {}'.format(possible_attr))
                  all_attr.append(None)
  
          return all_attr
docs: return the attributes for each channels.
  
          Parameters
          ----------
          attr : str
              attribute specified in Chan.attr.keys()