I'm using the Pool function of the multiprocessing module in order to run the same code in parallel on different data.
It turns out that on some data my code raises an exception, but the precise line in which this happens is not given:
Traceback (most recent call last):
File "my_wrapper_script.py", line 366, in <module>
main()
File "my_wrapper_script.py", line 343, in main
results = pool.map(process_function, folders)
File "/usr/lib64/python2.6/multiprocessing/pool.py", line 148, in map
return self.map_async(func, iterable, chunksize).get()
File "/usr/lib64/python2.6/multiprocessing/pool.py", line 422, in get
raise self._value
KeyError: 'some_key'
I am aware of multiprocessing.log_to_stderr() , but it seems that it is useful when concurrency issues arise, which is not my case.
Any ideas?
If you're using a new enough version of Python, you'll actually see the real exception get printed prior to that one. For example, here's a sample that fails:
import multiprocessing
def inner():
raise Exception("FAIL")
def f():
print("HI")
inner()
p = multiprocessing.Pool()
p.apply(f)
p.close()
p.join()
Here's the exception when running this with python 3.4:
multiprocessing.pool.RemoteTraceback:
"""
Traceback (most recent call last):
File "/usr/local/lib/python3.4/multiprocessing/pool.py", line 119, in worker
result = (True, func(*args, **kwds))
File "test.py", line 9, in f
inner()
File "test.py", line 4, in inner
raise Exception("FAIL")
Exception: FAIL
"""
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "test.py", line 13, in <module>
p.apply(f)
File "/usr/local/lib/python3.4/multiprocessing/pool.py", line 253, in apply
return self.apply_async(func, args, kwds).get()
File "/usr/local/lib/python3.4/multiprocessing/pool.py", line 599, in get
raise self._value
Exception: FAIL
If using a newer version isn't an option, the easiest thing to do is to wrap your worker function in a try/except block that will print the exception prior to re-raising it:
import multiprocessing
import traceback
def inner():
raise Exception("FAIL")
def f():
try:
print("HI")
inner()
except Exception:
print("Exception in worker:")
traceback.print_exc()
raise
p = multiprocessing.Pool()
p.apply(f)
p.close()
p.join()
Output:
HI
Exception in worker:
Traceback (most recent call last):
File "test.py", line 11, in f
inner()
File "test.py", line 5, in inner
raise Exception("FAIL")
Exception: FAIL
Traceback (most recent call last):
File "test.py", line 18, in <module>
p.apply(f)
File "/usr/local/lib/python2.7/multiprocessing/pool.py", line 244, in apply
return self.apply_async(func, args, kwds).get()
File "/usr/local/lib/python2.7/multiprocessing/pool.py", line 558, in get
raise self._value
Exception: FAIL
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