Default `warnings.showwarning` always used when interpreter session is cleaning up

Bug report

Bug description:

Hey!

I'm not sure this is a bug, exactly, or just behaviour that is not documented well (or at all, despite me looking I couldn't really find anything on it), but I recently ran into it and was scratching my head for a good hour.

The "issue" if it even is one, is that the default warnings.showwarning handler is always called when the main scope for the interpreter is getting cleaned up, even if there is a custom showwarning hook.

I noticed this by having some resource leak messages in the __del__ calls for some objects, and noticed that they only trigger the proper handler if they are triggered in any scope under the main scope, but if done so in the main scope they always use the default handler.

The following code reproduces the behaviour:

import warnings

def _warning_handler(message, category, filename, lineno, output_file, line):
    warn = warnings.formatwarning(message, category, filename, lineno, line)
    print(f'😺: {warn}', file = output_file)

warnings.showwarning = _warning_handler

def meow():
    print(f'Warning handler is: {warnings.showwarning.__code__.co_filename}')
    warnings.warn('meow')

class Cat:
    def __del__(self):
        print(f'Warning handler is: {warnings.showwarning.__code__.co_filename}')
        warnings.warn('nya')

meow()

cat = Cat()

The output of this code is:

Warning handler is: /tmp/mvp.py
😺: /tmp/mvp.py:11: UserWarning: meow
  warnings.warn('meow')

Warning handler is: /tmp/mvp.py
/tmp/mvp.py:16: UserWarning: nya

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-150944

從 issue 中的重現腳本開始，將其正常警告路徑與直譯器清理行為進行比較。查看連結的 PR gh-150944，了解提議的範圍；完成的標準是自訂的 warnings.showwarning 處理程序在清理過程中始終得到遵循，並為重現腳本提供涵蓋。