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

[Weatherby,Gerard]

Really doesn’t have much to do with Python and very much with standard streams, which go back as far as the 1950s. https://en.wikipedia.org/wiki/Standard_streams

When a user types -h / --help to a Python argparse script, the output of the script is the help message. The standard behavior is to print that and exit, regardless of any other arguments that are passed. So its behavior is consistent with the concept of standard out.

From: Python-list <python-list-bounces+gweatherby=uchc.edu at python.org> on behalf of MRAB <python at mrabarnett.plus.com>
Date: Tuesday, January 3, 2023 at 11:41 AM
To: python-list at python.org <python-list at python.org>
Subject: Re: What should go to stdout/stderr and why Python logging write everything to stderr?
*** Attention: This is an external email. Use caution responding, opening attachments or clicking on links. ***

On 2023-01-03 15:35, 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.

The purpose of stderr is to display status messages, logging and error
messages, even user prompts, and not mess up the program's actual
output. This is important on a *nix system where you might be piping the
output of one program into the input of another.
--
https://urldefense.com/v3/__https://mail.python.org/mailman/listinfo/python-list__;!!Cn_UX_p3!jni-tegcOXxLEkj6ohH0wbjz0Gd2OmHirJln0S1hX5z3nT6MUnqw4-ZMgaJGkzT228O9zSczGws4tyRERCMntNGR$<https://urldefense.com/v3/__https:/mail.python.org/mailman/listinfo/python-list__;!!Cn_UX_p3!jni-tegcOXxLEkj6ohH0wbjz0Gd2OmHirJln0S1hX5z3nT6MUnqw4-ZMgaJGkzT228O9zSczGws4tyRERCMntNGR$>