code: def _update(self, *_):
          
          if getattr(self, '_unlinked', False):
              return
          if getattr(self, '_updating', False):
              return
          self._updating = True
          try:
              setattr(self.target[0], self.target[1], self.transform(
                  getattr(self.source[0], self.source[1])
              ))
          finally:
              self._updating = False
docs: Set target value to source value