Bug report

Bug description:

Currently, "pre-finalization" (callbacks executed while the interpreter is still fully intact) looks like this:

Threads are joined first, then pending calls are executed, and finally atexit callbacks are executed. The problem is that any of these three can create one another, such as a pending call creating a thread, or an atexit handler scheduling a pending call. The best way to demonstrate this is by creating a new thread inside of an atexit callback:

import atexit
import threading
import time


def run():
    print(24)
    time.sleep(1)
    print(42)

@atexit.register
def start_thread():
    threading.Thread(target=run).start()

On 3.13+ (and probably earlier), you'll get one of these three outputs:

1. 24 is printed.
2. Nothing is printed.
3. A fatal error occurs due to the stdout lock being unavailable.

This is because the thread created is incorrectly non-daemon, because the call to threading._shutdown (which should join it) has already happened.

@ericsnowcurrently, I discussed this with you at the PyCon sprints. I think we came to the agreement that the best way to fix this is by looping until all three of them execute no callbacks.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-136003: Execute pre-finalization callbacks in a loop #136004