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If you instantiate a ThreadPool, then call map() with an empty list, the join() method will block indefinitely on self._result_handler.join()
$ python
> from multiprocessing.pool import ThreadPool
> t = ThreadPool(1)
> t.map_async(lambda x: x, [])
> t.close()
> t.join() # <- never returns
I was not able to determine the root cause, however, I found that the join() blocks when joining the _result_handler thread.