QSortProxyFilterModel.data converts object to list

My python object that has some definitions like a list but is much bigger than just that.

The object contains __len__(), __contains__(), __getitem__().

QSortProxyFilterModel.data(index, Qt.UserRole) returns the object as [] instead of the expected Asset type object.

Tested with other objects that contain no __getitem__() etc. They work as expected.
After converting __getitem__() to a normal method like get_asset() fixes this issue. However I prefer __getitem__().