pytest.warns(None) does not fail if warnings are raised.

[Jasha10]

I expect that pytest.warns(None) is an assertion that no warnings are raised.

import warnings
import pytest

def test_no_warning():
    with pytest.warns(None):
        warnings.warn("Look out!")

Running pytest on the above does not fail. I would expect a failure, something like "a warning was raised where no warning was expected".

$ pip list
Package    Version
---------- -------------------
attrs      21.2.0
certifi    2020.12.5
iniconfig  1.1.1
packaging  20.9
pip        21.0.1
pluggy     0.13.1
py         1.10.0
pyparsing  2.4.7
pytest     6.2.4
setuptools 52.0.0.post20210125
toml       0.10.2
wheel      0.36.2
$ python --version
Python 3.9.4

OS: Ubuntu 18.04

[Zac-HD]

See the docs:

To record with pytest.warns without asserting anything about the warnings, pass None as the expected warning type

This StackOverflow question shows how to assert that no warning was raised.

[Jasha10]

Awesome, thanks! Your answer to that SO question looks good :)