def METHOD_NAME(self, proxyIndex):
    """returns index into original model"""
    if proxyIndex is None:
        return None
    if proxyIndex.isValid():
        r2, c2, p2 = self.proxy_to_source(proxyIndex.row(), proxyIndex.column())
        sourceIndex = self.sourceModel().index(
            r2, c2, parent=p2
        )  # self.sourceModel().root_node[r2]
    else:
        sourceIndex = QtCore.QModelIndex()
    return sourceIndex