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'

import traceback
import multiprocessing.managers


class MyManager(multiprocessing.managers.SyncManager):
    pass

class DictList(multiprocessing.managers.BaseProxy):
    _method_to_typeid_ = {'__getitem__': 'dict'}

    def __getitem__(self, key):
        return self._callmethod('__getitem__', (key,))

MyManager.register('DictList', None, DictList)

with MyManager() as manager:

    nested = manager.DictList([{'hello': 'world'}])
    print(nested[0]['hello'])  # world

    proxy = manager.list([nested])
    try:
        print(proxy[0][0]['hello'])
    except AttributeError:
        traceback.print_exc()
        print("""
        Bug: AttributeError: ProxyBase._callmethod is None
        --------------------------------------------------

        This error is raised because proxies returned as #RETURN messages
        have no manager reference, which is required to resolve typeids
        (using BaseManager._registry).

        Only proxies returned as #PROXY messages will be fully functional.

        This is an undocumented current implementation limitation.

        Fix (proposal)
        --------------

        Include the manager class (not the instance) as part of the proxy
        serialization in BaseProxy.__reduce__, as BaseManager._registry is
        a class variable.

        Note: #PROXY message protocol can be also replaced by regular proxy
              serialization after this fix, resulting on simpler codebase.
        """)