Finding exception in python multiprocessing
multiprocessing, multithreading, python, stack-trace
Solution
I went back in my git history until I found a commit where things were still working.
I added a class to my code that extends `dict` so that keys can be accessed with a `.` (so `dict.foo` in stead of `dict["foo"]`. Multiprocessing did not take kindly to this, using an ordinary dict solved the problem.
Problem
I have a bit of python code that looks like this: ``` procs = cpu_count()-1 if serial or procs == 1: results = map(do_experiment, experiments) else: pool = Pool(processes=procs) results = pool.map(do_experiment, experiments) ``` It runs fine when I set the `serial` flag, but it gives the following error when the `Pool` is used. When I try to print something from `do_experiment` nothing shows up, so I can't try/catch there and print a stack trace. ``` Exception in thread Thread-2: Traceback (most recent call last): File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 530, in __bootstrap_inner self.run() File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 483, in run self.__target(*self.__args, **self.__kwargs) File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/pool.py", line 285, in _handle_tasks put(task) TypeError: 'NoneType' object is not callable ``` What is a good way to proceed debugging this?