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

Bắt đầu với script tái hiện trong issue và so sánh đường đi cảnh báo thông thường của nó với hành vi dọn dẹp của trình thông dịch. Xem lại PR được liên kết gh-150944 để xem phạm vi đề xuất; được coi là hoàn tất khi handler warnings.showwarning tùy chỉnh luôn được tôn trọng trong quá trình dọn dẹp, với coverage cho reproducer.