Hi Zsolt,
A few small things.
In the non index case the new find_all_inheritors call returns the
same list as the one right below it. You could reuse it there.
For the docs I would write "are skipped with a warning".
The last test, the parent with only foreign partitions, goes through
the same code path as the one before it. I would drop it.
Otherwise it looks good to me.
Thanks
Shihao