+ @property
+ def is_done(self) -> bool:
+ """Wrapper around self._is_done so we can control its setter."""
+ return self._is_done
+
+ @is_done.setter
+ def is_done(self, value: bool) -> None:
+ if value != self.is_done and not self.is_doable:
+ raise BadFormatException('cannot change doneness of undoable Todo')
+ self._is_done = value
+