warnings: support for logging.captureWarnings

[blueyed]

Moved from #2445 (comment):

The warnings plugin does not handle the case where the logging module's captureWarnings mechanism is used to route warnings through the logging module, and therefore allows to filter them:

logging.captureWarnings(True)
logging.getLogger('py.warnings').addFilter(filter_deprecation_warnings)

logging.captureWarnings monkeypatches warnings.showwarning (in Python 3.6) to log them instead (as a warning).
pytest could maybe check if logging._warnings_showwarning is not None to detect if warnings are supposed to go through logging.

With or without this in setup.cfg:

[tool:pytest]
filterwarnings =
    once::DeprecationWarning
    once::PendingDeprecationWarning

This mechanism is useful if you want to have fine-grained support on filtering, e.g. based on some (dynamic) path in the warning message.

I am not sure if it is worth the effort to do something in this regard, and if so what the best option would be: maybe transferring it into a log record and see if the py.warnings handlers would filter them?

[nicoddemus]

Thanks for posting the issue @blueyed.

IIUC the idea is for pytest to no longer capture warnings if logging.captureWarnings is set, IOW, just let the logging module handle warnings itself?

Just to make sure I understand the issue: if you disable the warnings plugin using -p no:warnings, the logging methods works as usual correct?

[blueyed]

Yes, and yes.

It is really a corner case, and I am going to remove this from the affected project anyway, but rather rely on what filtering warnings provides (via pytest's settings also).

[nicoddemus]

Would it be sufficient to mention this as a blurb in the docs, or do you feel we need additional support in the code?

[blueyed]

Don't let this hold back a release, not even for the blurb in the docs, please.

I will have to think myself (and would appreciate any input of course) about how this should get handled / documented: after all you want to filter warnings by redirecting them through logging.

The issue here is that warnings.catch_warnings (used by pytest) disables that.
So it might even be considered to be an issue in the stdlib - depending on where you look at it from. I can see that you really might want to catch all warnings with that.

[nicoddemus]

Don't let this hold back a release, not even for the blurb in the docs, please.

Docs should not hold a release, given that they are published directly from master. 😉

I will have to think myself (and would appreciate any input of course) about how this should get handled / documented: after all you want to filter warnings by redirecting them through logging.

I think just documenting this use case in warnings.rst might be enough, something like a blurb/section stating to disable the plugin if you are using this feature from logging.