Allow control over log channels captured/displayed by CLI

[SeanDS]

What's the problem this feature will solve?

My application emits a lot of logs. I'd like to view only a certain channel or channels when I specify e.g. --log-level=debug in the pytest CLI.

Describe the solution you'd like

It would be nice to be able to specify --log-channel=myapp.a.b one or more times to display only certain channels. The default could be --log-channel=* to display everything, but, if a non-default value is given, only the specified channels should be emitted to the console.

This would allow me to filter out log noise from irrelevant channels, especially at the debug level of my application, when trying to debug a specific failing test.

Alternative Solutions

I've not thought of any other solutions. I could add a log handler to my code directly, but that would defeat the purpose of configuring this on-demand via the CLI.

[RonnyPfannschmidt]

logging does not have "channels" i presume you mean prefixes in the logger tree?

[SeanDS]

logging does not have "channels" i presume you mean prefixes in the logger tree?

I mean the name of the logger used to emit log messages, i.e. the name parameter passed to logging.getLogger().

[twmr]

It is possible to silence certain loggers by using sth like

    logging.getLogger("PIL").setLevel(logging.WARNING)
    logging.getLogger("matplotlib").setLevel(logging.WARNING)

in your conftest.py. The other direction, i.e., enabling only specific loggers, is not easily possible AFAIK.

Wouldn't disabling noisy loggers suffice your needs @SeanDS?

[SeanDS]

@Thisch thanks for the tip. I am able to work around the noisy log channels already with code like the one you posted - but given how nicely the pytest CLI supports other logging features, when I wanted to filter out these log channels I went looking for a flag and didn't find one. It just feels to me like there should be such a flag in the CLI.

There are a few ways I imagine this could be implemented. Easiest might be to change the behaviour in catching_logs to simply add the log handler to the specified log prefixes instead of the root logger if --log-channel (or perhaps better, --log-prefix) is specified one or more times, i.e. could change:

    def __enter__(self):
        root_logger = logging.getLogger()
        if self.level is not None:
            self.handler.setLevel(self.level)
        root_logger.addHandler(self.handler)
        if self.level is not None:
            self.orig_level = root_logger.level
            root_logger.setLevel(min(self.orig_level, self.level))
        return self.handler

to something like:

    def __enter__(self):
        prefixes = self.log_prefixes if self.log_prefixes else [None]
        for prefix in prefixes:
            prefix_logger = logging.getLogger(prefix)
            if self.level is not None:
                self.handler.setLevel(self.level)
            prefix_logger.addHandler(self.handler)
            if self.level is not None:
                self.orig_level = prefix_logger.level
                prefix_logger.setLevel(min(self.orig_level, self.level))
        return self.handler

Obviously self.log_prefixes would need filled by the arg parser and __exit__ would need changed too, but could that be enough to make this work?