What should go to stdout/stderr and why Python logging write everything to stderr?

[Richard Damon]



> On Jan 3, 2023, at 10:38 AM, c.buhtz at posteo.jp wrote:
> Hello,
> 
> this posting isn't about asking for a technical solution. My intention
> is to understand the design decision Python's core developers made in
> context of that topic.
> 
> The logging module write everything to stderr no matter which logging
> level is used.
> 
> The argparse module does it more differentiated. If arguments are
> mandatory but none are given then argparse produce a short version of
> the usage info; on stderr. If the user request the usage info via "-h"
> it goes to stdout. This is the behavior I would expect.
> 
> Why does logging behave different? DEBUG and INFO imho should go to
> stdout not stderr.
> 
> Of course I could modify the handlers etc. to workaround this. But I
> assume that there was something in mind of the Python developers when
> they decided that.
> 
> My goal is not to divide between the use of print() or logging.info()
> in my own code. This would mess up a lot.
> -- 
> https://mail.python.org/mailman/listinfo/python-list

My thought are, that a “Log” should be a COMPLETE output of what is logged. Logged messages are NOT intended to be a message to the current user, but for a post operation analysis. Remember, a message sent by logging.info may never be seen, as that level of logging may be turned off. (You are providing ways to control how much is being set to the log, especially if going to the “screen”).

As such, stderr is a often better spot to send the log than stdout, as the program may well be using stdout to interact with the user, and you don’t want the interaction cluttered with the logging.

Also, I am fairly sure you can configure the logger to do what you want by just configuring two different logging handlers, one for stderr that does just errors, and one for stdout that shows all messages. If you don’t want the errors on stderr and stdout, just log the errors to a different logger which is on an error.* hierarchy and don’t let that hierarchy propagate up to the root logger.