Should warn when using `pytest_plugins` in a test file (since it's also not the root conftest.py)

• a detailed description of the bug or problem you are having
• output of pip list from the virtual environment you are using
• pytest and operating system versions
• minimal example if possible

Description

If I load a plugin using pytest_plugins in a test file, pytest should issue a warning similar to the warning given when using pytest_plugins in a non-root conftest.py.

I'm opening this as a bug because this seems unintentional. When pytest started discouraging pytest_plugins in non-root conftest.py, it should have been discouraged in test files too. If folks would prefer this as a feature request, I'm happy to change it.

Why this is a problem

The same issues with plugins in non-root conftest.py : The plugin gets activated globally for the rest of the test run, even though it seems like it'd only be activated for that one test file. (#3625 and #2062 have some discussion on pitfalls of pytest_plugins outside root conftest.py)

Moreover, since there's now a warning/error when using pytest_plugins in non-root conftest.py, the lack of a warning when using pytest_plugins in test files implies that using pytest_plugins in test files doesn't have the same problems.

Of course, the problems still apply to test files. From my experiments (using pytest 8.3.4):

• plugin fixtures are available in all test files (even if requested before running the test file with pytest_plugins)
• autouse fixtures are used in all tests that execute after the test file with pytest_plugins executes
• hooks get registered/run in all test files (including the ones run before the file with pytest_plugins)

And then if the file with pytest_plugins is omitted from the test run (e.g., by selecting a specific file), then the plugin is not loaded at all. (This behavior is reasonable/good, but it can cause surprising/confusing test failures when combined with plugins in test files.)

Minimal example

# content of tests/some_plugin.py

import pytest

@pytest.fixture()
def some_fixture():
    return "some value"

# content of tests/test_1_before_plugin.py

def test_some_fixture(some_fixture):
    assert some_fixture == "some value"

# content of tests/test_2_load_plugin.py

pytest_plugins = ["tests.some_plugin"]

def test_some_fixture(some_fixture):
    assert some_fixture == "some value"

# content of tests/test_3_after_plugin.py

def test_some_fixture(some_fixture):
    assert some_fixture == "some value"

$ pytest tests

Other info

• pytest: 8.3.4
• OS: NixOS

$ pip list
Package                Version Editable project location
---------------------- ------- ----------------------------------------------------------
attrs                  24.2.0
fancycompleter         0.9.1
iniconfig              2.0.0
packaging              24.2
pdbpp                  0.10.3
pip                    23.2.1
pluggy                 1.5.0
Pygments               2.18.0
pyrepl                 0.9.0
pytest                 8.3.4
pytest-fixture-scoping 0.1.0   [...]/bug-repros/pytest-fixture-scoping
setuptools             68.2.0
wmctrl                 0.5

Reproduce the behavior with tests/test_1_before_plugin.py, tests/test_2_load_plugin.py, and tests/test_3_after_plugin.py, using pytest tests as the first run. Trace how pytest_plugins is handled in a test file and compare it with the existing warning for non-root conftest.py. Done means a warning is emitted for the test-file case and coverage verifies the documented plugin-loading behavior.