Daphne disables warnings module

[mjnaderi]

Hi. I'm using django-channels in a django project. I wanted to use Python's warnings module to display a warning message. But it didn't work. Nothing was printed. I started the server with python -Wa manage.py runserver, but the warning didn't show up.

My code:

import warnings

def myview(request):
    warnings.warn("you've been warned!")
    ...

I removed channels from INSTALLED_APPS and ran runserver again. Issue was gone and the message was printed.

I tracked down the issue to this line in daphne code (server.py):

# Redirect the Twisted log to nowhere
globalLogBeginner.beginLoggingTo(
    [lambda _: None], redirectStandardIO=False, discardBuffer=True
)

In beginLoggingTo, twisted runs this code:

self._warningsModule.showwarning = self.showwarning

which replaces warnings.showwarning with a custom function. (When I comment this line in twisted, issue is fixed.)

Would you please show me how can I use Python's warnings module properly?

[mjnaderi]

Versions:

daphne: 3.0.1
channels: 3.0.2
django: 3.1.4
twisted: 20.3.0

[carltongibson]

Hi @mjnaderi — initial ideal would be to run with verbosity 1 (or greater).

[mjnaderi]

Hi @carltongibson. I'm running the project using manage.py runserver, and channels.management.commands.runserver doesn't pass verbosity argument to Server, so it runs with default verbosity=1 (verbosity >=2 is required to fix the issue).

I think this issue can make a lot of confusion for users. It's changing the default behavior of warnings module. All runtime warning messages from all packages (even Django itself!) are lost, and it's hard to find the reason.

[mjnaderi]

I tried increasing verbosity (by manually modifying server.py). Increasing verbosity alone can't solve the issue. I needed to modify Django's LOGGING to make it work.

LOGGING = {
    ...
    "loggers": {
        "daphne": {
            "handlers": ["console"]
        }
    },
}

[carltongibson]

OK, so at the least we could do with some docs here. Passing verbosity would also be worthwhile. I need to have a look into WHY the behaviour is as it is before deciding how/if we can change it. Happy if you want to have a dig though!

Thanks for the report!

[mjnaderi]

I did some experiment on this. I didn't find out why twisted redirects warnings to log messages. But I implemented a simple work-around: mjnaderi/channels@13fbe8a

[carltongibson]

Hi @mjnaderi — glad you found a workaround. If you want to make a PR to Channel's runserver that would be very welcome.

[mjnaderi]

Thanks @carltongibson
I opened a PR: django/channels#1620

[mjnaderi]

@carltongibson any update on this?