import multiprocessing
import multiprocessing.managers

def f(n):
    return n * n

def worker(pool):
    with pool:
        pool.apply_async(f, (10, ))

manager = multiprocessing.managers.SyncManager()
manager.start()
pool = manager.Pool(processes=4)
proc = multiprocessing.Process(target=worker, args=(pool, ))
proc.start()
proc.join()

This is related to BPO-35917 and it fails with:

Process Process-2:
Traceback (most recent call last):
  File "/home/giampaolo/cpython/Lib/multiprocessing/process.py", line 302, in _bootstrap
    self.run()
  File "/home/giampaolo/cpython/Lib/multiprocessing/process.py", line 99, in run
    self._target(*self._args, **self._kwargs)
  File "foo.py", line 54, in worker
    pool.apply_async(f, (10, ))
  File "<string>", line 2, in apply_async
  File "/home/giampaolo/cpython/Lib/multiprocessing/managers.py", line 802, in _callmethod
    proxytype = self._manager._registry[token.typeid][-1]
AttributeError: 'NoneType' object has no attribute '_registry'