Description
Binding the return value of the pytest.raises context manager to a variable prevents some warnings from being triggered as part of the test. As a result, the warning is not displayed in the test run summary, but only on stderr, and -W error will not fail the test.
Reproducible example:
import asyncio
import pytest
async def my_task():
pass
def test_scheduler_must_be_created_within_running_loop() -> None:
with pytest.raises(RuntimeError) as _:
asyncio.create_task(my_task())
Running the above code:
$ pytest -s -W error
===== test session starts =====
platform linux -- Python 3.10.8, pytest-7.1.3, pluggy-1.0.0
rootdir: /tmp/t
collected 1 item
test_a.py .
===== 1 passed in 0.00s =====
sys:1: RuntimeWarning: coroutine 'my_task' was never awaited
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
When replacing with pytest.raises(RuntimeError) as _: with with pytest.raises(RuntimeError):, everything works as expected and the warning causes the test run to fail.
My understanding is that the coroutine triggers the warning when it gets garbage collected. It looks like with pytest.raises as _ keeps a reference to the coroutine that should trigger the warning. The reference prevents garbage collection of the coroutine until after the hook wrappers in unraisableexception have returned. As a result, the warning appears outside of the test case.
Additional information
$ pip list
Package Version
---------- -------
attrs 22.1.0
iniconfig 1.1.1
packaging 21.3
pip 22.3
pluggy 1.0.0
py 1.11.0
pyparsing 3.0.9
pytest 7.1.3
setuptools 65.4.1
tomli 2.0.1