logging should cache LogRecord.message

Bug report

Bug description:

logging currently does not cache LogRecord.message:
https://github.com/python/cpython/blob/0dcbc8385322ff51f7fc3e586027d880275df4fa/Lib/logging/__init__.py#L391-L401
https://github.com/python/cpython/blob/0dcbc8385322ff51f7fc3e586027d880275df4fa/Lib/logging/__init__.py#L711

Since logging caches other string-formatting related results such as LogRecord.asctime and LogRecord.exc_text, I would like to suggest:

1. LogRecord.getMessage should check self.message first. If it is not computed yet, it should compute the message and set this attribute before returning.
2. Formatter shouldn't set record.message. It only needs to call LogRecord.getMessage to get the message.

How to reproduce

import logging

class A:
    def __repr__(self):
        print('repr called')
        return '<A>'

logger = logging.getLogger()
logger.addHandler(logging.StreamHandler())
logger.addHandler(logging.StreamHandler())
logger.error('%r', A())

Expected output

repr called
<A>
<A>

Actual output

repr called
<A>
repr called
<A>

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-134367

Lisez Lib/logging/init.py aux emplacements référencés de LogRecord et Formatter, puis exécutez la reproduction pour observer les appels répétés à repr. Examinez le PR lié gh-134367 avant de commencer. C’est terminé lorsque l’exemple à deux handlers appelle repr une seule fois tandis que les deux handlers produisent le message.