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You say "for testing purposes only", but I'd add "for debugging" to the list of use-cases, having just spent a few hours trying to understand why I was getting a particular PySide2 "Internal C++ object already deleted." exception under Python 3.6 but not under Python 3.7 and later. [*]
Even just making the list of exit handlers introspectable (so just _get_exitfuncs()) would have been really useful in diagnosing the issue faster.
[*] The answer turned out to be that under Python 3.6 the PySide2 exit handler was being run before the concurrent.futures thread-cleanup exit handler, while on Python 3.7, for obscure reasons, the exit handlers were being registered in the reverse order. To discover this, I ended up resorting to the elegant hack (if that's not a contradiction in terms) described in https://stackoverflow.com/a/63029332/270986