testing python multiprocessing pool code with nose

Question

I am trying to write tests with nose that get set up with something calculated using multiprocessing.

I have this directory structure:

code/
    tests/
        tests.py

tests.py looks like this:

import multiprocessing as mp


def f(i):
    return i ** 2


pool = mp.Pool()
out = pool.map(f, range(10))


def test_pool():
    """Really simple test that relies on the output of pool.map.
    The actual tests are much more complicated, but this is all
    that is needed to produce the problem."""
    ref_out = map(f, range(10))
    assert out == ref_out

if __name__ == '__main__':
    test_pool()

Running from the code directory, python tests/tests.py passes.

nosetests tests/tests.py fails to complete. It starts up, but never gets through the call to pool.mapand just hangs.

Why is this and what is the simplest solution?

Answer 1

The problem is related to the fact that pool.map is called at the "global level". Normally you want to avoid that, because these statements will be executed even if your file is simply imported.

Nose has to import your module to be able to find your tests and later execute them, therefore I believe the problem happens while the import mechanism kicks in (I haven't spent time trying to find out the exact reason for this behaviour)

You should move your initialization code to a test fixture instead; Nose supports fixtures with the with_setup decorator. Here is one possibility (probably the simplest change while keeping pool and out as globals):

import multiprocessing as mp
from nose import with_setup

pool = None
out  = None

def f(i):
    return i ** 2

def setup_func():
    global pool
    global out
    pool = mp.Pool()
    out  = pool.map(f, range(10))

@with_setup(setup_func)
def test_pool():
    """Really simple test that relies on the output of pool.map.
    The actual tests are much more complicated, but this is all
    that is needed to produce the problem."""
    global out
    ref_out = map(f, range(10))
    assert out == ref_out

if __name__ == '__main__':
    test_pool()

Executing:

$ nosetests tests/tests.py
.
----------------------------------------------------------------------
Ran 1 test in 0.011s

OK