from concurrent.futures import ThreadPoolExecutor, wait
executor = ThreadPoolExecutor()
futures = ()
for i in range(25):
    futures += executor.submit(lambda i=i: print(i)),
wait(futures)

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Traceback (most recent call last):
  File "thread.py", line 9, in <module>
    futures += executor.submit(lambda: print(i)),
  File "/usr/local/lib/python3.6/concurrent/futures/thread.py", line 115, in submit
    self._adjust_thread_count()
  File "/usr/local/lib/python3.6/concurrent/futures/thread.py", line 134, in _adjust_thread_count
    t.start()
  File "/usr/local/lib/python3.6/threading.py", line 846, in start
    _start_new_thread(self._bootstrap, ())
RuntimeError: can't start new thread