Finding the source of an exception in a python multiprocessing program

[Dave Angel]

On 04/24/2013 05:09 PM, William Ray Wing wrote:
> On Apr 24, 2013, at 4:31 PM, Neil Cerutti <neilc at norwich.edu> wrote:
>
>> On 2013-04-24, William Ray Wing <wrw at mac.com> wrote:
>>> When I look at the pool module, the error is occurring in
>>> get(self, timeout=None) on the line after the final else:
>>>
>>>     def get(self, timeout=None):
>>>         self.wait(timeout)
>>>         if not self._ready:
>>>             raise TimeoutError
>>>         if self._success:
>>>             return self._value
>>>         else:
>>>             raise self._value
>>
>> The code that's failing is in self.wait. Somewhere in there you
>> must be masking an exception and storing it in self._value
>> instead of letting it propogate and crash your program. This is
>> hiding the actual context.
>>
>> --
>> Neil Cerutti
>> --
>> http://mail.python.org/mailman/listinfo/python-list
>
> I'm sorry, I'm not following you.  The "get" routine (and thus self.wait) is part of the "pool" module in the Python multiprocessing library.
> None of my code has a class or function named "get".
>
> -Bill
>

My question is why bother with multithreading?  Why not just do these as 
separate processes?  You said "they in no way interact with each other" 
and that's a clear clue that separate processes would be cleaner.

Without knowing anything about those libraries, I'd guess that somewhere 
they do store state in a global attribute or equivalent, and when that 
is accessed by both threads, it can crash.

Separate processes will find it much more difficult to interact, which 
is a good thing most of the time.  Further, they seem to be scheduled 
more efficiently because of the GIL, though that may not make that much 
difference when you're time-limited by network data.

-- 
DaveA