def _update(self, *_):
        """Set target value to source value"""
        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
Set target value to source value