Known issues

This section describes the known issues in MPIRE.

Unit tests

When using the 'spawn' or 'forkserver' method you’ll probably run in to one or two issues when running unittests in your own package. One problem that might occur is that your unittests will restart whenever the piece of code containing such a start method is called, leading to very funky terminal output. To remedy this problem make sure your setup call in setup.py is surrounded by an if __name__ == '__main__': clause:

from setuptools import setup

if __name__ == '__main__':

    # Call setup and install any dependencies you have inside the if-clause
    setup(...)

See the ‘Safe importing of main module’ section at caveats.

The second problem you might encounter is that the semaphore tracker of multiprocessing will complain when you run individual (or a selection of) unittests using python setup.py test -s tests.some_test. At the end of the tests you will see errors like:

Traceback (most recent call last):
  File ".../site-packages/multiprocess/semaphore_tracker.py", line 132, in main
    cache.remove(name)
KeyError: b'/mp-d3i13qd5'
.../site-packages/multiprocess/semaphore_tracker.py:146: UserWarning: semaphore_tracker: There appear to be 58
                                                         leaked semaphores to clean up at shutdown
  len(cache))
.../site-packages/multiprocess/semaphore_tracker.py:158: UserWarning: semaphore_tracker: '/mp-f45dt4d6': [Errno 2]
                                                         No such file or directory
  warnings.warn('semaphore_tracker: %r: %s' % (name, e))
...

Your unittests will still succeed and run OK. Unfortunately, I’ve not found a remedy to this problem using python setup.py test yet. What you can use instead is something like the following:

python -m unittest tests.some_test

This will work just fine. See the unittest documentation for more information.

Unpicklable tasks/results

Sometimes you can encounter deadlocks in your code when using MPIRE. When you encounter this, it could well be that some tasks or results from your script can’t be pickled. MPIRE makes use of multiprocessing queues for inter-process communication and if your function returns unpicklable results the queue will unfortunately deadlock.

The only way I could remedy this problem in MPIRE would be to manually pickle objects before sending it to a queue and quit gracefully when encountering a pickle error. However, this would mean objects would always be pickled twice. This would add a heavy performance penalty and is therefore not an acceptable solution. Instead, the user should make sure their tasks and results are always picklable (which in most cases won’t be a problem).