Pytest will not issue PytestAssertRewriteWarning when __pycache__ is used

• description:

When a test contains an always true boolean there is a warning that the assert is always true:

: PytestAssertRewriteWarning: assertion is always true, perhaps remove parentheses?

This is as expected for the given example.

When the test is re run (using the cache generated in the first run) there is no warning given, this means that they can be missed and the test appears to pass.

Setting environment variable

PYTHONDONTWRITEBYTECODE=1

and invalidating/deleting pycache allows the warning to reappear

• OS affected = all

• pytest version = 6.2.0, 6.2.4

• pip list

Package Version

astroid 2.4.2
attrs 20.3.0
boto3 1.16.47
botocore 1.19.63
certifi 2020.12.5
chardet 4.0.0
h5py 3.2.1
idna 2.10
iniconfig 1.1.1
isort 5.7.0
jmespath 0.10.0
lazy-object-proxy 1.4.3
mccabe 0.6.1
more-itertools 8.6.0
numpy 1.20.3
packaging 20.9
pip 21.1.1
pluggy 0.13.1
py 1.10.0
pylint 2.6.2
pyparsing 2.4.7
pytest 6.2.0
python-dateutil 2.8.1
pytz 2021.1
requests 2.25.1
s3transfer 0.3.4
setuptools 51.1.2
six 1.15.0
toml 0.10.2
urllib3 1.26.3
wheel 0.36.2
wrapt 1.12.1

minimal example:

def test_this_warns(): assert (1 == 2, "you will never read this")

• 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

Run the minimal test example twice with normal bytecode caching enabled, then repeat after deleting pycache or setting PYTHONDONTWRITEBYTECODE=1. Compare the PytestAssertRewriteWarning output between runs and trace the assertion-rewriting cache path. Done means the warning is issued consistently when cached bytecode is used.