Exception thrown in multiprocessing Pool not detected

Question

It seems that when an exception is raised from a multiprocessing.Pool process, there is no stack trace or any other indication that it has failed. Example:

from multiprocessing import Pool 

def go():
    print(1)
    raise Exception()
    print(2)

p = Pool()
p.apply_async(go)
p.close()
p.join()

prints 1 and stops silently. Interestingly, raising a BaseException instead works. Is there any way to make the behavior for all exceptions the same as BaseException?

Answer 1

Maybe I'm missing something, but isn't that what the get method of the Result object returns? See Process Pools.

class multiprocessing.pool.AsyncResult

The class of the result returned by Pool.apply_async() and Pool.map_async().get([timeout])
Return the result when it arrives. If timeout is not None and the result does not arrive within timeout seconds then multiprocessing.TimeoutError is raised. If the remote call raised an exception then that exception will be reraised by get().

So, slightly modifying your example, one can do

from multiprocessing import Pool

def go():
    print(1)
    raise Exception("foobar")
    print(2)

p = Pool()
x = p.apply_async(go)
x.get()
p.close()
p.join()

Which gives as result

1
Traceback (most recent call last):
  File "rob.py", line 10, in <module>
    x.get()
  File "/usr/lib/python2.6/multiprocessing/pool.py", line 422, in get
    raise self._value
Exception: foobar

This is not completely satisfactory, since it does not print the traceback, but is better than nothing.

UPDATE: This bug has been fixed in Python 3.4, courtesy of Richard Oudkerk. See the issue get method of multiprocessing.pool.Async should return full traceback.