def METHOD_NAME(self, sourceIndex):
    """returns index into proxy model"""
    if sourceIndex is None:
        return None
    if sourceIndex.isValid():
        r2, c2, p2 = self.source_to_proxy(
            sourceIndex.row(), sourceIndex.column(), sourceIndex.parent()
        )
        proxyIndex = self.index(r2, c2, p2)
    else:
        proxyIndex = QtCore.QModelIndex()
    return proxyIndex