def METHOD_NAME(self, proxyIndex: QModelIndex) -> QModelIndex:
    if not proxyIndex.isValid():
        return QModelIndex()
    sm = self.sourceModel()
    iter_index = sm.index(self._iter, 0, QModelIndex())
    if not iter_index.isValid() or not sm.hasChildren(iter_index):
        return QModelIndex()
    real_index = sm.index(proxyIndex.row(), proxyIndex.column(), iter_index)
    return real_index