importlib.resources regression for custom ResourceReader

Since Python 3.13 (including 3.14), my custom ResourceReader is now raising errors when used with importlib.resources.contents. This code is otherwise functioning fine from 3.8 onwards.

Traceback (most recent call last):
  File "D:\cpython\t.py", line 41, in <module>
    print(contents(FakeModule))
          ~~~~~~~~^^^^^^^^^^^^
  File "D:\cpython\Lib\importlib\resources\_functional.py", line 60, in contents
    return (resource.name for resource in _get_resource(anchor, path_names).iterdir())
                                          ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
  File "D:\cpython\Lib\importlib\resources\_functional.py", line 81, in _get_resource
    return files(anchor).joinpath(*path_names)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
TypeError: CompatibilityFiles.SpecPath.joinpath() missing 1 required positional argument: 'other'

Self-contained repro below (the real one gets compiled into DLLs, so you don't want to try to debug it ;) ):

from importlib.abc import Loader
from importlib.machinery import ModuleSpec
from importlib.resources.abc import ResourceReader

_DATA = {"FakeModule.NAME": b"hello"}
_DATA_NAMES = set(_DATA)

class MyReader(ResourceReader):
    def __init__(self, prefix):
        self.prefix = prefix

    def open_resource(self, resource):
        import io
        return io.BytesIO(_DATA[self.prefix + resource])

    def resource_path(self, resource):
        raise FileNotFoundError()

    def is_resource(self, resource):
        return self.prefix + resource in _DATA_NAMES

    def contents(self):
        p = self.prefix
        lp = len(p)
        return (n[lp:] for n in _DATA_NAMES if n.startswith(p))

class MyLoader(Loader):
    create_module = ...
    exec_module = ...

    def get_resource_reader(self, fullname):
        return MyReader(fullname + ".")

class FakeModule:
    __loader__ = MyLoader()
    __name__ = "FakeModule"
    __spec__ = ModuleSpec(__name__, __loader__)

from importlib.resources import contents
print(contents(FakeModule))

(ping @jaraco)

Linked PRs

• gh-154919