pytest failed to parse the filterwarnings option due to subtle issues related to python path

env

pytest: 7.2.0
system: macOS Sonaoma 14.7.1
python: 3.10.9

Problem description

1. create a project folder,
   create mod1.py:

import warnings


class MyWarning(UserWarning):
    pass


def useful_func():
    warnings.warn(
        MyWarning("some message"),
        stacklevel=1,
    )
    print("this is useful func")

    return 1

create test_mod1.py under the project

import sys
print(sys.path)

from mod1 import useful_func

def test_useful_func():
    assert 1 == useful_func()

2. run pytest -s under the project and you see the test pass, but with a warning
3. create a pyproject.toml under the project directory:

[tool.pytest.ini_options]
filterwarnings = [
"ignore::mod1.MyWarning",
]

4. run pytest again, you see the following error when pytest is trying to parse the warning filter:

ERROR: while parsing the following warning configuration:

  ignore::mod1.MyWarning

This error occurred:

Traceback (most recent call last):
  File "/opt/homebrew/Caskroom/miniconda/base/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1690, in parse_warning_filter
    category: Type[Warning] = _resolve_warning_category(category_)
  File "/opt/homebrew/Caskroom/miniconda/base/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1728, in _resolve_warning_category
    m = __import__(module, None, None, [klass])
ModuleNotFoundError: No module named 'mod1'

Additional info

I think the error is because when pytest is trying to parse the filterwarnings option, it does not add the project root to the sys.path yet. In a later step of pytest startup, the project root is eventually added to sys.path, which you can see from the step 2 above (when pyproject.toml is not used).

One way to fix the error is to change the warning filter a bit, e.g., to match the message instead:

[tool.pytest.ini_options]
filterwarnings = [
# "ignore::mod1.MyWarning",
"ignore:some message"
]

but this just evade the issue.

Another way is to run pytest via python -m pytest without changing the filter format in pyproject.toml. So I think it is a time issue related to when current directory is added to sys.path.

Not sure if this is intended or really a bug, thanks for reading!