+ def add_child(self, child: Todo) -> None:
+ """Add child to self.children, guard against recursion"""
+ def walk_steps(node: Todo) -> None:
+ if node.id_ == self.id_:
+ raise BadFormatException('bad child choice causes recursion')
+ for child in node.children:
+ walk_steps(child)
+ if self.id_ is None:
+ raise HandledException('Can only add children to saved Todos.')
+ if child.id_ is None:
+ raise HandledException('Can only add saved children to Todos.')
+ if child in self.children:
+ raise BadFormatException('cannot adopt same child twice')
+ walk_steps(child)
+ self.children += [child]
+