On Mon, Mar 9, 2020 at 11:18 PM Kyle Stanley <aeros167@gmail.com> wrote:
We're currently looking into adjusting the current Executor implementation (for both TPE and PPE) to not use daemon threads at all for subinterpreter compatibility, see https://bugs.python.org/issue39812. Instead of using an atexit handler, there's some consideration for using a similar threading-specific "exit handler". Instead of being upon program exit, registered functions would be called just before all of the non-daemon threads are joined in `threading._shutdown()`.

As a result, I suspect that we are unlikely to implement something like the above. But, as a convenient side effect, it should somewhat address the current potentially surprising behavior.
There might still be some fun involved given that current implementation *might* lazy-spawn threads upon submit.
This means that if a ThreadExecutor is called from both daemon and non-daemon threads (which is a very bad idea though), the kind of spawned threads might be unpredictable.