It's only in imap, in map it's ok. The following code explains the issue:

from multiprocessing.pool import ThreadPool

pool = ThreadPool(10)

def gen():
    yield 1 + '1' # here is an error

try:
next((pool.imap(str, gen())))
except:
# Will be catched using pool.map normally, but using pool.imap will be not.
# Instead it hangs. This is the same for ThreadPool and Pool.
print('this will not be printed because thread is hanging')

Using the supplied example, I was unable to reproduce what you described using either 3.5 or 3.6-beta on OS X 10.11.

What platform are you using? (Perhaps it is platform specific.)

Ubuntu 14.04 LTS, 3.13.0-83-generic, x86_64

So, I've checked twice, it's presented by me on python 3.4.3, and not by 3.5.2. So I will go deaper

I've reproduced it on 2 different machines:

• on a MacBook in Docker (debian:jessie, python 3.4.2)
• on another desktop (Ubuntu 14.04.1, 3.16.0-77-generic, x86_64, python 3.4.3)

It's hanging in a while loop in _handle_workers, /usr/lib/python3.4/multiprocessingpool.py:365.
I can't figure out what is the reason.

@staticmethod
def _handle_workers(pool):
    thread = threading.current_thread()

    # Keep maintaining workers until the cache gets drained, unless the pool
    # is terminated.
    while thread._state == RUN or (pool._cache and thread._state != TERMINATE):
        pool._maintain_pool()
        time.sleep(0.1)
    # send sentinel to stop workers
    pool._taskqueue.put(None)
    util.debug('worker handler exiting')

If it only occurs in 3.4.x, is moving to 3.5 an option for you?

Yes, I'm free to move to 3.5, now I'm seeing isn't there any problems in 3.5 according to this issue.

In Py3.6, it raises error:

>>> next((pool.imap(str, gen())))
Traceback (most recent call last):
  File "/opt/lib/python3.7/multiprocessing/pool.py", line 684, in next
    item = self._items.popleft()
IndexError: pop from an empty deque

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/lib/python3.7/multiprocessing/pool.py", line 690, in next
    item = self._items.popleft()
IndexError: pop from an empty deque

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/lib/python3.7/multiprocessing/pool.py", line 693, in next
    raise StopIteration
StopIteration

But this doesn't looks good. I think it should raise TypeError. It can be fixed in bpo-28699.

Closing this issue -- all further discussion moves to bpo-28699