Undocumented breaking change for str vs pathlib.Path usage between v8.x and v9 in Config.args?

[RHammond2]

In a few libraries I manage, I use flags to manage running just unit or regression tests using some shorthand arguments that get run via pytest --unit or pytest --regression. This collects the test files from the directories I want to run. In up to v9, I have been able to directly pass PosixPath objects to config.args, however, there seems to be a behavior change because I now have to pass a str for all the arguments.

This was a bit of a confusing change because the error in the example below isn't very obvious, and doesn't seem to be directly documented anywhere. The fix was ultimately an easy fix to just convert the paths to strings, but I'm wondering also if this was an intended change somewhere along the lines that I've missed, a missing deprecation, or bug?

Exact code from the simplest instance of the behavior is directly pulled from https://github.com/WISDEM/WOMBAT/blob/main/tests/conftest.py.

from pathlib import Path

import pytest


ROOT = Path(__file__).parent
TEST_DATA = ROOT / "library"


def pytest_addoption(parser):  # noqa: D103
    parser.addoption(
        "--unit", action="store_true", default=False, help="run tests in test/unit/."
    )
    parser.addoption(
        "--regression",
        action="store_true",
        default=False,
        help="run tests in test/regression/.",
    )


def pytest_configure(config):  # noqa: D103
    # Check for the options
    unit = config.getoption("--unit")
    regression = config.getoption("--regression")

    # Provide the appropriate directories
    unit_tests = [el for el in (ROOT / "unit").iterdir() if el.suffix == ".py"]
    regression_tests = [
        el for el in (ROOT / "regression").iterdir() if el.suffix == ".py"
    ]

    # If both, run them all; if neither skip any modifications; otherwise run just the
    # appropriate subset
    if regression and unit:
        config.args = unit_tests + regression_tests
    elif regression:
        config.args = regression_tests
    elif unit:
        config.args = unit_tests

Running pytest works just fine, but once I use either the --unit or --regression flags, I get the following output.

==================================================================================== test session starts =====================================================================================
platform darwin -- Python 3.13.7, pytest-9.0.1, pluggy-1.6.0
cachedir: .cache
rootdir: /Users/rhammond/GitHub_Public/NREL/WOMBAT
configfile: pyproject.toml
testpaths: tests
plugins: anyio-4.11.0, check-2.6.0, cov-7.0.0
collected 0 items                                                                                                                                                                            
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/_pytest/main.py", line 318, in wrap_session
INTERNALERROR>     session.exitstatus = doit(config, session) or 0
INTERNALERROR>                          ~~~~^^^^^^^^^^^^^^^^^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/_pytest/main.py", line 371, in _main
INTERNALERROR>     config.hook.pytest_collection(session=session)
INTERNALERROR>     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/pluggy/_hooks.py", line 512, in __call__
INTERNALERROR>     return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
INTERNALERROR>            ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/pluggy/_manager.py", line 120, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
INTERNALERROR>            ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/pluggy/_callers.py", line 167, in _multicall
INTERNALERROR>     raise exception
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/pluggy/_callers.py", line 139, in _multicall
INTERNALERROR>     teardown.throw(exception)
INTERNALERROR>     ~~~~~~~~~~~~~~^^^^^^^^^^^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/_pytest/logging.py", line 788, in pytest_collection
INTERNALERROR>     return (yield)
INTERNALERROR>             ^^^^^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/pluggy/_callers.py", line 139, in _multicall
INTERNALERROR>     teardown.throw(exception)
INTERNALERROR>     ~~~~~~~~~~~~~~^^^^^^^^^^^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/_pytest/warnings.py", line 98, in pytest_collection
INTERNALERROR>     return (yield)
INTERNALERROR>             ^^^^^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/pluggy/_callers.py", line 139, in _multicall
INTERNALERROR>     teardown.throw(exception)
INTERNALERROR>     ~~~~~~~~~~~~~~^^^^^^^^^^^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/_pytest/config/__init__.py", line 1372, in pytest_collection
INTERNALERROR>     return (yield)
INTERNALERROR>             ^^^^^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/pluggy/_callers.py", line 121, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/_pytest/main.py", line 382, in pytest_collection
INTERNALERROR>     session.perform_collect()
INTERNALERROR>     ~~~~~~~~~~~~~~~~~~~~~~~^^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/_pytest/main.py", line 814, in perform_collect
INTERNALERROR>     resolve_collection_argument(
INTERNALERROR>     ~~~~~~~~~~~~~~~~~~~~~~~~~~~^
INTERNALERROR>         self.config.invocation_params.dir,
INTERNALERROR>         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR>     ...<3 lines>...
INTERNALERROR>         consider_namespace_packages=consider_namespace_packages,
INTERNALERROR>         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR>     )
INTERNALERROR>     ^
INTERNALERROR>   File "/Users/rhammond/miniconda3/envs/wombat-test/lib/python3.13/site-packages/_pytest/main.py", line 1116, in resolve_collection_argument
INTERNALERROR>     base, squacket, rest = arg.partition("[")
INTERNALERROR>                            ^^^^^^^^^^^^^
INTERNALERROR> AttributeError: 'PosixPath' object has no attribute 'partition'