Textualize/textual · error · TypeError

`after` argument must be an index or a TreeNode object to ad

Error message

`after` argument must be an index or a TreeNode object to add after

What it means

TypeError from TreeNode.add(): the `after` argument is neither an int index nor a TreeNode, so no insertion position can be derived.

Source

Thrown at src/textual/widgets/_tree.py:422

            else:
                raise TypeError(
                    "`before` argument must be an index or a TreeNode object to add before"
                )

        if after is not None:
            if isinstance(after, int):
                insert_index = after + 1
                if after < 0:
                    insert_index += len(self.children)
            elif isinstance(after, TreeNode):
                try:
                    insert_index = self.children.index(after) + 1
                except ValueError:
                    raise AddNodeError(
                        "The node specified for `after` is not a child of this node"
                    )
            else:
                raise TypeError(
                    "`after` argument must be an index or a TreeNode object to add after"
                )

        text_label = self._tree.process_label(label)
        node = self._tree._add_node(self, text_label, data)
        node._expanded = expand
        node._allow_expand = allow_expand
        self._updates += 1
        self._children.insert(insert_index, node)
        self._tree._invalidate()

        return node

    def add_leaf(
        self,
        label: TextType,
        data: TreeDataType | None = None,
        *,

View on GitHub (pinned to 06dbeef4bb)

Solutions

1. Convert to int: `after=int(value)`.
2. Pass the TreeNode instead of an identifier string.

Example fix

# before
node.add("item", after="1")
# after
node.add("item", after=int("1"))

if not isinstance(after, (int, TreeNode)):
    after = None
node.add(label, after=after)

from textual.widgets._tree import TreeNode
def is_position(n: object) -> bool:
    return isinstance(n, (int, TreeNode))

try:
    node.add(label, after=pos)
except TypeError:
    node.add(label)

When it happens

Trigger: `node.add("x", after=3.5)`, `after="2"`, `after=(1,)` or any non-int/non-TreeNode value.

Common situations: Index read from user input or JSON as a string; refactor changed the expected type.

Related errors

• `before` argument must be an index or a TreeNode object to a
• Unable to add a node both before and after a node
• The node specified for `before` is not a child of this node
• The node specified for `after` is not a child of this node
• {self.name} must be a str

AI-assisted analysis of Textualize/textual@06dbeef4bb (2026-08-27). Data as JSON: /api/errors/b269009793c6020c. Report an issue: GitHub.