pytest thread safety should be better documented

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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()

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

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

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.

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.

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. 👍

Add thread safety section to flaky test docs (#12359)

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pytest thread safety should be better documented #12356

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pytest thread safety should be better documented #12356

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The-Compiler commented on May 23, 2024

RonnyPfannschmidt commented on May 23, 2024

ngoldbaum commented on May 23, 2024

ngoldbaum commented on May 23, 2024

nicoddemus commented on May 23, 2024

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