[MRG+1] Remove warnings in nested Parallel when the inner Parallel has n_jobs=1

[harishmk]

suppress a warning if a daemonic process creates parallel jobs with n_jobs=1
#405

[GaelVaroquaux]

LGTM. +1 for merge. Thank you!

[aabadie]

I could reproduce the initial problem mentionned in #405 on Windows with python 2.7. The fix looks good to me.
One thing is missing though : a test. Can you add a test function in joblib/test/test_parallel.py that checks no warning is raised when n_jobs = 1 ?

[lesteve]

A snippet reproducing the problem:

from joblib import Parallel, delayed


def func():
    return 42


def inner_parallel_func():
    Parallel(n_jobs=1)(delayed(func)() for _ in range(10))

if __name__ == '__main__':
    Parallel(n_jobs=2)(delayed(inner_parallel_func)() for _ in range(10))

[lesteve]

I used this as an excuse to test the recent features of github where PR authors can allow the maintainers to push commits on the PR branch and it seems to work just fine, this is very nifty!

I added a test for the warnings, as you can see from the diff it's not that easy to catch the warnings because they happen in child processes so I reuse and enhanced check_subprocess_call ...

[lesteve]

@aabadie do you want to have a look at this one before I merge?

[aabadie]

not super critical but I think this comment is useless or there's something missing after.

[lesteve]

good point, I tried different ways of flushing stderr to get the number of warnings I expected to no avail so I eventually gave up, except I forgot the comment there.

[aabadie]

I admit adding a test for this was a bit tricky :)

[lesteve]

I admit adding a test for this was a bit tricky :)

I was quite happy that check_subprocess_call (originally here for checking that a function defined in __main__ can be used inside Parallel, which is very useful in interactive i.e. IPython settings) could be reused without too much pain for the test.

[aabadie]

Merging this one. Thanks @harishmk and @lesteve

[lesteve]

@harishmk do you mind giving me your full name ?

I would need it for CHANGES.rst since I am planning to do a bug-fix release of joblib (mainly for scikit-learn 0.18.1) and this PR would be part of it. If I don't hear from you, I'll just use your github user name.