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> Are you sure? I didn't see the daemon flag in ThreadPoolExecutor.
Oh you're right, ThreadPoolExecutor does create daemon threads:
class ThreadPoolExecutor(_base.Executor):
 ...
 def _adjust_thread_count(self):
 # When the executor gets lost, the weakref callback will wake up
 # the worker threads.
 def weakref_cb(_, q=self._work_queue):
 q.put(None)
 # TODO(bquinlan): Should avoid creating new threads if there are more
 # idle threads than items in the work queue.
 if len(self._threads) < self._max_workers:
 t = threading.Thread(target=_worker,
 args=(weakref.ref(self, weakref_cb),
 self._work_queue))
 t.daemon = True <======= HERE
 t.start()
 self._threads.add(t)
 _threads_queues[t] = self._work_queue