Warnings plugin's "once" filtering not working as expected

[renefritze]

This is probably related to #1043, but that was reported before the pytest_warnings plugin was merged into pytest proper.

What I'm seeing is a warning recorded for every realization of a parameterized fixture instead of only one.

Now given this mwe.py:

import warnings
import pytest
import pprint

@pytest.fixture(params=[1,2,3])
def fixture(request):
    return request.param

def test_me(fixture):
    if fixture < 2:
        pprint.pprint(warnings.filters)
    warnings.warn('some text', DeprecationWarning)

if __name__ == '__main__':
    for i in range(1,4):
        test_me(i)

I expected pytest -s -W once::DeprecationWarning mwe.py to print exactly one notice for the warning.
I get all three, however:

 ============================= test session starts =============================
platform linux -- Python 3.5.3, pytest-3.1.3, py-1.4.34, pluggy-0.4.0
rootdir: /tmp, inifile:
collected 3 items 
 
mwe.py [('once',
  re.compile('', re.IGNORECASE),
  <class 'DeprecationWarning'>,
  re.compile(''),
  0),
 ('ignore', None, <class 'DeprecationWarning'>, None, 0),
 ('ignore', None, <class 'PendingDeprecationWarning'>, None, 0),
 ('ignore', None, <class 'ImportWarning'>, None, 0),
 ('ignore', None, <class 'BytesWarning'>, None, 0),
 ('ignore', None, <class 'ResourceWarning'>, None, 0),
 ('ignore',
  re.compile('', re.IGNORECASE),
  <class 'pkg_resources.PEP440Warning'>,
  re.compile(''),
  0)]
...

============================== warnings summary ===============================
mwe.py::test_me[1]
  /tmp/mwe.py:12: DeprecationWarning: some text
    warnings.warn('some text', DeprecationWarning)

mwe.py::test_me[2]
  /tmp/mwe.py:12: DeprecationWarning: some text
    warnings.warn('some text', DeprecationWarning)

mwe.py::test_me[3]
  /tmp/mwe.py:12: DeprecationWarning: some text
    warnings.warn('some text', DeprecationWarning)

-- Docs: http://doc.pytest.org/en/latest/warnings.html
==================== 3 passed, 3 warnings in 0.01 seconds =====================

Running python -W once::DeprecationWarning mwe.py shows what I was expecting:

[('once',
  re.compile('', re.IGNORECASE),
  <class 'DeprecationWarning'>,
  re.compile(''),
  0),
 ('ignore', None, <class 'DeprecationWarning'>, None, 0),
 ('ignore', None, <class 'PendingDeprecationWarning'>, None, 0),
 ('ignore', None, <class 'ImportWarning'>, None, 0),
 ('ignore', None, <class 'BytesWarning'>, None, 0),
 ('ignore', None, <class 'ResourceWarning'>, None, 0)]
mwe.py:12: DeprecationWarning: some text
  warnings.warn('some text', DeprecationWarning)

edit: improved formatting

[nicoddemus]

Thanks @renemilk for the report!

[Zac-HD]

@asottile - I would expect #4104 to have fixed this, but I just reproduced it with pytest 3.9.1 😕

Any idea what's happening?

[asottile]

#4104 only changed behavior of recwarn on python 2.x so this is probably somewhere else in the stack