Parametrizing a test with an empty list calls `ids` callback with the private constant `NOTSET`

[mon]

I parametrize tests based on a user-provided list. Sometimes this list is empty, which disables some tests as a result. However, I would expect the ids callback that I provide to not be called at all in that case, but it is actually called with the private NOTSET sentinel. As this is not exported, I can't easily check for it in my ids function and return a useless value to match it.

Reproduction:

conftest.py

import logging

def idmap(obj):
    logging.info(obj)

def pytest_generate_tests(metafunc):
    if "arg" in metafunc.fixturenames:
        metafunc.parametrize(
            "arg", [], ids=idmap
        )

test_demo.py

def test_demo(arg):
    pass

$: pytest --collect-only --log-cli-level=DEBUG
...
INFO     root:conftest.py:4 NotSetType.token
...

Python: 3.10.15
pytest: 8.3.4
OS: macOS Sonoma 14.6.1

[RonnyPfannschmidt]

For empty id lists we generate a single test with a warning

[mon]

Yes, which is reasonable - but I can't detect this in my ID mapping function because it's a hidden type. Perhaps the sentinel can just be exported? Or don't call the ID map for the "no parametrization" case.

I can make a PR in either case!

[RonnyPfannschmidt]

https://github.com/pytest-dev/pytest/blob/main/src%2F_pytest%2Fmark%2Fstructures.py#L182-L186

[mon]

I don't quite understand - I don't get access to the ParameterSet, and thus can't see its marks. I simply get NOTSET passed to the ids callback. Because NOTSET is internal to Pytest, I cannot compare the arg against it nor type annotate my ids function to accept it.

[RonnyPfannschmidt]

indeed - we should use a string that indicates it

[RonnyPfannschmidt]

i have a wip pr that sets the string NOTSET as direct id to avoid this - 'll polish it a litlle and push

[mon]

Fantastic, thank you so much!