finalizer.py

import gc
import time


class Message:
    def __init__(self):
        self._exception = None

    def __del__(self):
        print("running!")


def f():
    raise RuntimeError("test")


def g():
    m = Message()
    d = {"a": m}  # putting something else on the stack with a reference to m
    try:
        f()
    except Exception as e:
        m._exception = e

g()
print("pre sleep 1")
time.sleep(1)
gc.collect()
print("after sleep 1")
time.sleep(1)
print("after sleep 2")

Yields:

python test.py
pre sleep 1
running!
after sleep 1
after sleep 2

on Python 3.8 on macOS.