Textualize/textual · error · AddNodeError

Unable to add a node both before and after a node

Error message

Unable to add a node both before and after a node

What it means

TreeNode.add() raises AddNodeError when both `before` and `after` keyword arguments are supplied. Position is ambiguous, so the API refuses to guess where to insert the new node.

Source

Thrown at src/textual/widgets/_tree.py:390

            label: The new node's label.
            data: Data associated with the new node.
            before: Optional index or `TreeNode` to add the node before.
            after: Optional index or `TreeNode` to add the node after.
            expand: Node should be expanded.
            allow_expand: Allow user to expand the node via keyboard or mouse.

        Returns:
            A new Tree node

        Raises:
            AddNodeError: If there is a problem with the addition request.

        Note:
            Only one of `before` or `after` can be provided. If both are
            provided a `AddNodeError` will be raised.
        """
        if before is not None and after is not None:
            raise AddNodeError("Unable to add a node both before and after a node")

        insert_index: int = len(self.children)

        if before is not None:
            if isinstance(before, int):
                insert_index = before
            elif isinstance(before, TreeNode):
                try:
                    insert_index = self.children.index(before)
                except ValueError:
                    raise AddNodeError(
                        "The node specified for `before` is not a child of this node"
                    )
            else:
                raise TypeError(
                    "`before` argument must be an index or a TreeNode object to add before"
                )

View on GitHub (pinned to 06dbeef4bb)

Solutions

1. Pass only one of `before` or `after`.
2. If position comes from a variable, default the other to None: `node.add(label, before=pos_before, after=pos_after)` where at least one is None.

Example fix

# before
node.add("item", before=2, after=3)
# after
node.add("item", before=2)

assert not (before is not None and after is not None), "pass only one of before/after"
node.add(label, before=before, after=after)

from textual.widgets._tree import AddNodeError
try:
    node.add(label, before=b, after=a)
except AddNodeError:
    node.add(label)  # append

When it happens

Trigger: Calling `node.add("label", before=x, after=y)` (or add_node with both) — both arguments must be None/int/TreeNode, and at most one may be given.

Common situations: Copy-pasting insertion code and forgetting to delete one keyword; dynamic code that passes a positioning dict like `node.add(label, **pos)` where pos contains both keys.

Related errors

• The node specified for `before` is not a child of this node
• `before` argument must be an index or a TreeNode object to a
• The node specified for `after` is not a child of this node
• `after` argument must be an index or a TreeNode object to ad
• Attempt to remove the root node of a Tree.

AI-assisted analysis of Textualize/textual@06dbeef4bb (2026-08-27). Data as JSON: /api/errors/26beb5b5f11a9f2e. Report an issue: GitHub.