Remove deprecated logging.Logger.warn() method in Python 3.13

[vstinner]

The logging.Logger.warn() method was deprecated in Python 3.3 by issue #57444 and commit 04d5bc0. This method is not documented and emits a DeprecationWarning since Python 3.3.

#57444 (comment):

That's deliberate. The original code (before incorporation into Python) had warn(), which was kept for backward compatibility. The docs refer to warning() because that's what everyone is supposed to use. The method names map to the lower case of the appropriate logging level name.

I propose to remove this logging.Logger.warn() method in Python 3.13.

Linked PRs

• gh-105376: Remove logging.Logger.warn() method #105377
• gh-105376: Remove logging.warn() and LoggerAdapter.warn() #106553
• gh-105376: Restore deprecated logging warn() method #122775
• [3.13] gh-105376: Restore deprecated logging warn() method (GH-122775) #122856

[vstinner]

cc @vsajip

The alternative is to remove the deprecation and keep this alias: see issue #105373.

[davidmayo]

I would like to speak up for reversing this change. A lot of people, especially non-professional programmers, have written a lot of code with logging.warn(), and it is going to break in a way that will be very confusing to them. It's going to be especially thorny, since the thing that will fail will be their logging utility itself, which is usually where they would go to begin troubleshooting.

If this really is going to be removed, I would recommend at least one Python release where using logging.warn() emits a SyntaxWarning instead of a DeprecationWarning before removing it entirely, so that people have at least some visibility of the problem before their code breaks. This was done for invalid escape sequences in string literals in Python3.12, and I think it's a good model. Release notes link.

EDIT: I didn't notice until after I posted that the SyntaxWarning for invalid escape sequences was also done by @vstinner !

[vsajip]

Whereas invalid escape sequences might reasonably be considered syntax warnings, I'm not sure that applies to methods that were deprecated long ago. In any case, it's easy enough for users to fix their broken code, in the (presumably) very rare instances where they didn't see the deprecation warning because they upgraded to the very latest Python version from a pre- 3.3 version.

Can you explain why you think a SyntaxWarning is better than the DeprecationWarning which was already in place?

[hugovk]

These can be found with the Ruff linter: https://docs.astral.sh/ruff/rules/#flake8-logging-format-g

For example:

ruff check --isolated --select G010 .

And auto-fixed:

ruff check --isolated --select G010 . --fix

And found with pylint: https://pylint.readthedocs.io/en/stable/user_guide/messages/warning/deprecated-method.html

pylint  --disable=all --enable=W4902 .

And a Flake8 plugin: https://pypi.org/project/flake8-logging-format/

[davidmayo]

@vsajip My preferred solution is to keep the warn() method in Logger with no changes.

If that isn't possible, I recommended the SyntaxWarning approach because I thought that it is visible by default, whereas DeprecationWarning is invisible by default. But based on some quick testing, that seems like it's not actually the case, at least universally.

I was trying to replicate this kind of behavior, where the warning about the deprecated behavior became more visible in 3.12:

❯ python3.11 -c "print('\d')"
\d

❯ python3.12 -c "print('\d')"
<string>:1: SyntaxWarning: invalid escape sequence '\d'
\d