Redirecting stderr when using multiprocessing.Pool - python-2.7

Question

I want to spawn multiple processes using multiprocessing.Pool (python 2.7.13), and redirect stdout / stderr of each process to a file. The problem is it works for stdout, but not for stderr. Here's an example with a single process.
import sys
import multiprocessing as mp
def foo():
sys.stdout = open('a.out','w')
sys.stderr = open('a.err', 'w')
print("this must go to a.out.")
raise Exception('this must go to a.err.')
return True
def run():
pool = mp.Pool(4)
_retvals = []
_retvals.append( pool.apply_async(foo) )
retvals = [r.get(timeout=10.) for r in _retvals]
if __name__ == '__main__':
run()
Running python stderr.py in terminal (of macbook) produces a.out with correct message ("this must go to a.out"). But it produces empty a.err, and the error message appears in terminal window instead.
If I don't use multiprocessing.Pool and directly run it in the main thread, it produces correct messages on both files. This means replacing run() with the following snippet:
def run():
foo()

Answer 1

When using Pools, unhandled exceptions are handled by the the main process. You should either redirect stderr in main(), or wrap your functions like this:
def foo():
sys.stdout = open('x.out', 'a')
sys.stderr = open('x.err', 'a')
try:
print("this goes to x.out.")
print("this goes to x.err.", file=sys.stderr)
raise ValueError('this must go to a.err.')
except:
traceback.print_exc()
raise # optional