searching for XML elements with position predicates does not work at all when namespaces are involved.

Bug report

Bug description:

Failure with namespaces:

Python 3.12.7 | packaged by conda-forge | (main, Oct  4 2024, 16:05:46) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import xml.etree.ElementTree as ET
>>> ET.XML('<a xmlns="http://www.example.com/default-schema"><b/><b/></a>').find("./b[1]", {"": "http://www.example.com/default-schema"})
>>> ET.XML('<a xmlns="http://www.example.com/default-schema"><b/><b/></a>').find("./b[last()]", {"": "http://www.example.com/default-schema"})
Traceback (most recent call last):
  File "/home/lukas/miniforge3/envs/roads/lib/python3.12/xml/etree/ElementPath.py", line 370, in iterfind
    selector = _cache[cache_key]
               ~~~~~~^^^^^^^^^^^
KeyError: ('./b[last()]', ('', 'http://www.example.com/default-schema'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/lukas/miniforge3/envs/roads/lib/python3.12/xml/etree/ElementPath.py", line 405, in find
    return next(iterfind(elem, path, namespaces), None)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/lukas/miniforge3/envs/roads/lib/python3.12/xml/etree/ElementPath.py", line 384, in iterfind
    selector.append(ops[token[0]](next, token))
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/lukas/miniforge3/envs/roads/lib/python3.12/xml/etree/ElementPath.py", line 315, in prepare_predicate
    raise SyntaxError("unsupported function")
SyntaxError: unsupported function
>>> ET.XML('<a xmlns="http://www.example.com/default-schema"><b/><b/></a>').find("./b", {"": "http://www.example.com/default-schema"})
<Element '{http://www.example.com/default-schema}b' at 0x750110149b20>
>>>

• searching by integer position yields None
• searching with the predicate [last()] fails with "unsupported function"

This works fine when no namespaces are involved:

\>>> ET.XML('<a><b/><b/></a>').find("./b[1]")
<Element 'b' at 0x7501101498a0>
>>> ET.XML('<a><b/><b/></a>').find("./b[last()]")
<Element 'b' at 0x7501101499e0>
>>> 

With namespaces I would expect similar results.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-132822

Start by reproducing the namespace examples with xml.etree.ElementTree, comparing positional and last() predicates with the behavior shown for unnamespaced XML. Trace the ElementTree path-search implementation used by find(); done means both predicates return the expected elements with a default namespace. A linked PR, gh-132822, already exists.