AtsushiSakai/PythonRobotics · error · ValueError

Child is not set

Error message

Child is not set

What it means

Raised by DecoratorNode.not_set_child_raise_error, called from tick. Decorators (inverter, retry, etc.) wrap exactly one child; ticking one whose child was never assigned is rejected.

Source

Thrown at MissionPlanning/BehaviorTree/behavior_tree.py:305

        return Status.SUCCESS


class DecoratorNode(Node):
    """
    Base class for all decorator nodes in a behavior tree.

    Decorator nodes modify the behavior of their child node.
    They must have a single child and can alter the status of the child node.
    """

    def __init__(self, name):
        super().__init__(name)
        self.type = NodeType.DECORATOR_NODE
        self.child = None

    def not_set_child_raise_error(self):
        if self.child is None:
            raise ValueError("Child is not set")

    def reset_children(self):
        self.child.reset()


class InverterNode(DecoratorNode):
    """
    Inverter node that inverts the status of its child node.

    Returns:
        - Returns SUCCESS if the child returns FAILURE
        - Returns FAILURE if the child returns SUCCESS
        - Returns RUNNING if the child returns RUNNING

    Examples:
        .. code-block:: xml

            <Inverter>

View on GitHub (pinned to 1fe4fb980f)

Solutions

1. Set the decorator's child (root.child = inner_node) before ticking.
2. When loading from XML, give the decorator tag exactly one child element.
3. Tree-validate after construction: assert decorator.child is not None.

Example fix

// before
inv = InverterNode('inv')
tree.tick()  # child never set

// after
inv = InverterNode('inv')
inv.child = my_action
tree.tick()

assert decorator.child is not None, f'{decorator.name} has no child'

def has_child(node) -> bool:
    return getattr(node, 'child', None) is not None

When it happens

Trigger: Creating an InverterNode (or other decorator), not setting .child (e.g. not calling set_child / not configuring via the XML builder), and ticking the tree.

Common situations: Programmatically building a tree and forgetting to attach the wrapped node, or XML where the decorator tag is empty.

Related errors

• Children are not set
• {root.name} Control node must have children
• {root.name} Decorator node must have exactly one child
• Node is not implemented
• Unknown status

AI-assisted analysis of AtsushiSakai/PythonRobotics@1fe4fb980f (2026-08-28). Data as JSON: /api/errors/0709ff3a1a77a14d. Report an issue: GitHub.