pytest suppresses object allocation info in ResourceWarnings when PYTHONTRACEMALLOC is enabled

[mgedmin]

Recent Python 3 versions (I think 3.7+?) show additional information in ResourceWarnings when you enable PYTHONTRACEMALLOC:

$ cat demo.py
def leak():
    x = open('/dev/null')


if __name__ == "__main__":
    leak()

$ export PYTHONWARNINGS=all
$ export PYTHONTRACEMALLOC=10
$ python3 demo.py
demo.py:6: ResourceWarning: unclosed file <_io.TextIOWrapper name='/dev/null' mode='r' encoding='UTF-8'>
  leak()
Object allocated at (most recent call last):
  File "demo.py", lineno 6
    leak()
  File "demo.py", lineno 2
    x = open('/dev/null')

However pytest suppresses this somehow:

$ cat test.py
import demo


def test_leak():
    demo.leak()

$ export PYTHONWARNINGS=all
$ export PYTHONTRACEMALLOC=10
$ pytest test.py
/home/mg/.local/pipx/venvs/pytest/lib/python3.8/site-packages/_pytest/compat.py:340: PytestDeprecationWarning: The TerminalReporter.writer attribute is deprecated, use TerminalReporter._tw instead at your own risk.
See https://docs.pytest.org/en/stable/deprecations.html#terminalreporter-writer for more information.
  return getattr(object, name, default)
============================= test session starts ==============================
platform linux -- Python 3.8.2, pytest-6.0.1, py-1.9.0, pluggy-0.13.1
rootdir: /home/mg/tmp
collected 1 item                                                               

test.py .                                                                [100%]

=============================== warnings summary ===============================
test.py::test_leak
  /home/mg/tmp/test.py:5: ResourceWarning: unclosed file <_io.TextIOWrapper name='/dev/null' mode='r' encoding='UTF-8'>
    demo.leak()

-- Docs: https://docs.pytest.org/en/stable/warnings.html
========================= 1 passed, 1 warning in 0.01s =========================

It's pretty much impossible to debug ResourceWarnings without seeing where the objects were created, so I'd love any suggestions.

(I have tried playing with pytest's command line options like -Wnone, --disable-warnings or -p no:warnings, but either I get the same uninformative ResourceWarning, or I don't get any warning at all. In none of the cases I managed to see the object allocation stack trace.)

[graingert]

Related: #5299

[asottile]

dupe #5676

[mgedmin]

That looks like a different bug to me? For me the ResourceWarning is captured and displayed, just without the full information that Python itself provides.

[asottile]

yeah, same as here: #5676 (comment)