pytest thread safety should be better documented

[ngoldbaum]

• 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

pytest.warns is a subclass of the standard library warnings.catch_warnings context manager, which is not thread safe (this is noted in the docs).

If it's not possible for pytest to have a thread-safe version, it would be nice if the docs for pytest.warns could note that it's implemented using warnings.catch_warnings and is not thread safe for the same reason.

pytest 8.2.1 on MacOS Sonoma.

Fails about 10-20% of the time on my machine:

import warnings
import threading


def raise_warning():
    warnings.warn(RuntimeWarning())


def test_pytest_warns():
    b = threading.Barrier(2)

    def catch_warning():
        b.wait()
        with pytest.warns(RuntimeWarning):
            raise_warning()

    task1 = threading.Thread(target=catch_warning)
    task2 = threading.Thread(target=catch_warning)

    task1.start()
    task2.start()
    task1.join()
    task2.join()

[The-Compiler]

pytest in its entirety is not intended to be thread-safe IIRC.

[RonnyPfannschmidt]

pytest.warns is implemented in terms of the stdlib catch-warnings helper - which is inherently not threadsave

[ngoldbaum]

pytest in its entirety is not intended to be thread-safe IIRC.

TIL

Best to avoid using pytest constructs inside a thread, then.

This fact could be emphasized more in the docs, I don't see any mention of threads at all besides the discussion about unhandled exceptions.

[ngoldbaum]

pytest.warns is implemented in terms of the stdlib catch-warnings helper - which is inherently not threadsave

Yes, I noted that in the issue description. The issue is a request for that fact to be noted in the docs, like the docs for the stdlib helper, if making it thread safe is out of scope for the library.

[nicoddemus]

Makes sense to note somewhere that pytest's constructs/helpers are not thread safe.

If somebody wants to take a look and update the docs, would love to review a PR. 👍