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Probably best demonstrated by example.
~~~~~~~~~~~~~~~~
import multiprocessing
class Myerror(ValueError):
 def __init__(self,a):
 self.a = a
 def __str__(self):
 return repr(self.a)
 
def foo(arg):
 raise Myerror(arg)
 
#foo("1") #<= this works fine, raises exception as expected
#But this breaks:
pool = multiprocessing.Pool(2)
pool.map(foo, ["1","2","3"])
~~~~~~~~~~~~~~~~
The result seen:
~~~~~~~~~~~~~~~~
Traceback (most recent call last):
 File "/usr/lib/python2.7/threading.py", line 551, in __bootstrap_inner
 self.run()
 File "/usr/lib/python2.7/threading.py", line 504, in run
 self.__target(*self.__args, **self.__kwargs)
 File "/usr/lib/python2.7/multiprocessing/pool.py", line 353, in _handle_results
 task = get()
TypeError: ('__init__() takes exactly 2 arguments (1 given)', <class '__main__.Myerror'>, ())
~~~~~~~~~~~~~~~~
At this point the application hangs. Worse: pressing ctrl-C shows a traceback and KeyboardInterrupt, but the worker keeps getting restarted, so it's impossible to stop. You have to go to another shell and do somthing like
 killall python
to terminate the program.
A real-world example (which caused me to track this down) is a CalledProcessError raised by subprocess.check_call
~~~~~~~~~~~~~~~~
import multiprocessing
import subprocess
def foo(arg):
 subprocess.check_call("nonexistent", shell=True)
 #raise subprocess.CalledProcessError(127, "nonexistent")
pool = multiprocessing.Pool(2)
pool.map(foo, ["1","2","3"])
~~~~~~~~~~~~~~~~
which fails in the same way:
~~~~~~~~~~~~~~~~
Traceback (most recent call last):
 File "/usr/lib/python2.7/threading.py", line 551, in __bootstrap_inner
 self.run()
 File "/usr/lib/python2.7/threading.py", line 504, in run
 self.__target(*self.__args, **self.__kwargs)
 File "/usr/lib/python2.7/multiprocessing/pool.py", line 353, in _handle_results
 task = get()
TypeError: ('__init__() takes at least 3 arguments (1 given)', <class 'subprocess.CalledProcessError'>, ())
~~~~~~~~~~~~~~~~
Behaviour tested on:
python 2.7.3 on Ubuntu 12.04
python 2.7.1 on OSX 10.7.5
Workaround: re-raise a parameter-less exception instead.
 try:
 ...
 except Exception as e:
 raise RuntimeError