What causes "{type(self).__name__} does not support inserting rows" in slint-ui/slint?

Use slint.ListModel, which implements insert_row/append.

slint-ui/slint · error · NotImplementedError

does not support inserting rows

Error message

{type(self).__name__} does not support inserting rows

What it means

The base Model class's insert_row default implementation raises NotImplementedError because a generic model cannot insert rows. Subclasses must override insert_row (and call notify_row_added) to support insertion.

Solutions

Use slint.ListModel, which implements insert_row/append.
In a custom model subclass, override insert_row to insert into the backing store and call self.notify_row_added(row, 1).
If the model is intentionally immutable, avoid calling insert_row and expose updates via property bindings instead.

Example fix

# before
class MyModel(slint.Model):
    def row_count(self): return len(self.data)

# after
class MyModel(slint.Model):
    def row_count(self): return len(self.data)
    def insert_row(self, row, value):
        self.data.insert(row, value)
        self.notify_row_added(row, 1)

Defensive patterns

Strategy: type-guard

Validate before calling

def supports_insert(model):
    return type(model).insert_row is not slint.Model.insert_row

Type guard

def supports_insert(model):
    return isinstance(model, slint.ListModel) or overrides(model, "insert_row")

Try / catch

try:
    model.insert_row(0, item)
except NotImplementedError:
    model = slint.ListModel([item] + existing)

Prevention

Use slint.ListModel when rows will be added at runtime.
Override insert_row + notify_row_added in custom mutable models.

When it happens

Trigger: Calling model.insert_row(index, value) on a model whose class does not override insert_row.

Common situations: Subclassing slint.Model for a read-only data source and later appending items from UI logic; passing a custom minimal model to a component that adds rows.

Related errors

AI-assisted analysis of slint-ui/slint@3a7e700487 (2026-09-16). Data as JSON: /api/errors/d80926df67e94f0a. Report an issue: GitHub.

Appendix: source

Thrown at api/python/slint/slint/models.py:78

        """Add a new row to the model with the provided value.
        The default implementation calls `insert_row` with the row count."""
        self.insert_row(self.row_count(), value)

    def remove_row(self, row: int) -> None:
        """Remove the row at the given index.
        Raises an exception when the model rejects the modification.
        The default implementation raises NotImplementedError. A model that
        supports removing rows should also call `notify_row_removed`."""
        raise NotImplementedError(
            f"{type(self).__name__} does not support removing rows"
        )

    def insert_row(self, row: int, value: T) -> None:
        """Insert a new row at the given index.
        Raises an exception when the model rejects the modification.
        The default implementation raises NotImplementedError. A model that
        supports inserting rows should also call `notify_row_added`."""
        raise NotImplementedError(
            f"{type(self).__name__} does not support inserting rows"
        )

    def notify_row_changed(self, row: int) -> None:
        """Call this method from a sub-class to notify the views that a row has changed."""
        super().notify_row_changed(row)

    def notify_row_removed(self, row: int, count: int) -> None:
        """Call this method from a sub-class to notify the views that
        `count` rows have been removed starting at `row`."""
        super().notify_row_removed(row, count)

    def notify_row_added(self, row: int, count: int) -> None:
        """Call this method from a sub-class to notify the views that
        `count` rows have been added starting at `row`."""
        super().notify_row_added(row, count)

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