code: def sort(self, section: int, order=None):
          attr = self.header[section]
          old_i, old_sort = self.sort_state
          self.beginResetModel()
          if section == old_i:
              self.collection.sort(attr, not old_sort)
              self.sort_state = (section, not old_sort)
          else:
              self.collection.sort(attr, True)
              self.sort_state = (section, True)
          self.endResetModel()