When using a TreeBuilder with a custom factory (returning a subclass of Element with some extra functionality) there is an issue with findall(). XPaths using an indexed predicate no longer work: the findall() result is always empty. The attached test script shows the problem. I have analysed this as far as finding that the issue is in ElementPath.prepare_predicate.<locals>.select(), the last one. Specifically, the statement elems = list(parent.findall(elem.tag)) always returns an empty list (if the Element class is subclassed). When using Python 2.7 everything works fine. When adding a findall() method to the Element subclass that simply calls Element.findall() everything also works fine. I suspect some issue with the _elementtree C implementation but I don't understand it.

Correction to the last line: When using Python 2.7 everything works fine. When adding a findall() method to the Element subclass that simply calls ElementPath.findall() everything also works fine. I suspect some issue with the _elementtree C implementation but I don't understand it.

 New changeset b11c5667f99c4f0018e3394c4d07c519d835671a by Serhiy Storchaka in branch 'master': bpo-34941: Fix searching Element subclasses. (GH-9766) https://github.com/python/cpython/commit/b11c5667f99c4f0018e3394c4d07c519d835671a 

 New changeset b1c800303e8458e00428ae66351ad492a503a46f by Miss Islington (bot) in branch '3.7': bpo-34941: Fix searching Element subclasses. (GH-9766) https://github.com/python/cpython/commit/b1c800303e8458e00428ae66351ad492a503a46f 

 New changeset 7c1c42b3209f1d2546daab6cd77f953eb255df6c by Serhiy Storchaka in branch '3.6': [3.6] bpo-34941: Fix searching Element subclasses. (GH-9766) (GH-9868) https://github.com/python/cpython/commit/7c1c42b3209f1d2546daab6cd77f953eb255df6c