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Race condition with warnings issued once directly and once inside pytest.warns #169
Description
I bumped into a slighty surprising behaviour in scikit-learn and it took me a bit of time to realize that the root cause is that the same warning is issued twice, once directly in the test function and once inside a pytest.warns context. There appears to be some race condition that makes the test fail when run with enough iterations. Not sure pytest-run-parallel can do much about it but I thought I would report it here first.
Wild-guessing, maybe per-module __warningregistry__ is still a global (in contrary the per-thread warnings.filters since Python 3.14 and python/cpython#130010)? My understanding is that per-module __warningregistry__ is used on top of the warnings.filter to decide whether to issue repeated warnings.
Here is a reproducer:
# test.py
import warnings
import pytest
import my_module
def test_pytest_warns():
my_module.function_that_warns()
with pytest.warns(UserWarning):
my_module.function_that_warns()# my_module.py
import warnings
def function_that_warns():
warnings.warn('bla')pytest test.py --parallel-threads 4 --iterations 100
Output:
============================================================================================================= test session starts ==============================================================================================================
platform linux -- Python 3.14.2, pytest-9.0.1, pluggy-1.6.0
rootdir: /tmp
plugins: run-parallel-0.8.0, cov-7.0.0, repeat-0.9.4, xdist-3.8.0
collected 1 item
Collected 1 items to run in parallel
test.py e [100%]
==================================================================================================================== ERRORS ====================================================================================================================
______________________________________________________________________________________________________ ERROR at call of test_pytest_warns ______________________________________________________________________________________________________
def test_pytest_warns():
my_module.function_that_warns()
> with pytest.warns(UserWarning):
^^^^^^^^^^^^^^^^^^^^^^^^^
E Failed: DID NOT WARN. No warnings of type (<class 'UserWarning'>,) were emitted.
E Emitted warnings: [].
test.py:10: Failed
=============================================================================================================== warnings summary ===============================================================================================================
test.py: 286 warnings
/tmp/my_module.py:4: UserWarning: bla
warnings.warn('bla')
-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
********************************************************************************************************** pytest-run-parallel report **********************************************************************************************************
All tests were run in parallel! 🎉
=========================================================================================================== short test summary info ============================================================================================================
PARALLEL FAILED test.py::test_pytest_warns - Failed: DID NOT WARN. No warnings of type (<class 'UserWarning'>,) were emitted.
======================================================================================================== 286 warnings, 1 error in 0.07s ========================================================================================================
Possible work-arounds:
- make sure to use
with pytest.warnsto capture the first warning as well - mark test as thread-unsafe
- use
--mark-warnings-as-unsafe
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[-]Race condition with warnings issued once directly, once inside pytest.warns[/-][+]Race condition with warnings issued once directly and once inside pytest.warns[/+]ngoldbaum commentedon Jan 27, 2026
Ping @nascheme - do you happen to have some context about
__warningregistry__?To give a little extra context - the test in the issue passes every time on my machine with
--iterations=2, but starts failing randomly with--iterations=4and then around--iterations=8will fail every time. So this is a race condition - I think in CPython?nascheme commentedon Jan 29, 2026
Yes, it's still a per-module global.
The registry is supposed to get cleared every time the filters change. Code in the Python implementation:
The global warnings lock is held when this code runs. However, I think that doesn't prevent a race like the following:
warn_explicit(). Since the filters version changed, clear registry.If 3 and 4 steps are swapped, you either get or don't get a warning.