Qt for Python (PySide/Shiboken)

Those are signal pairs, one goes before a data change, one goes after. See the table https://doc.qt.io/qt-6/qabstractitemmodel.html#signals. Most are emit implicitly by the protected methods, for example beginInsertColumns()/endInsertColumns(). Notably, layout(AboutToBe)Changed has no such method pair. A well behaved model emits both, so that UIs can adjust accordingly. Maybe the UI wants to look at removed things, before the model deletes them, etc.