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

I have a reasonable solution for the problem, at least for debugging purposes. I do not currently have a solution that will raise the exception back in the main processes. My first thought was to use a decorator, but you can only pickle functions defined at the top level of a module, so that’s right out.

Instead, a simple wrapping class and a Pool subclass that uses this for apply_async (and hence apply). I’ll leave map_async as an exercise for the reader.

import traceback
from multiprocessing.pool import Pool
import multiprocessing

# Shortcut to multiprocessing's logger
def error(msg, *args):
    return multiprocessing.get_logger().error(msg, *args)

class LogExceptions(object):
    def __init__(self, callable):
        self.__callable = callable

    def __call__(self, *args, **kwargs):
        try:
            result = self.__callable(*args, **kwargs)

        except Exception as e:
            # Here we add some debugging help. If multiprocessing's
            # debugging is on, it will arrange to log the traceback
            error(traceback.format_exc())
            # Re-raise the original exception so the Pool worker can
            # clean up
            raise

        # It was fine, give a normal answer
        return result

class LoggingPool(Pool):
    def apply_async(self, func, args=(), kwds={}, callback=None):
        return Pool.apply_async(self, LogExceptions(func), args, kwds, callback)

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

multiprocessing.log_to_stderr()
p = LoggingPool(processes=1)

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

This gives me:

1
[ERROR/PoolWorker-1] Traceback (most recent call last):
  File "mpdebug.py", line 24, in __call__
    result = self.__callable(*args, **kwargs)
  File "mpdebug.py", line 44, in go
    raise Exception()
Exception