FIX treat n_jobs=None as if left to its default value

[jeremiedbb]

Fixes #1473

Even if the new default value of n_jobs is a sentinel object, I think that we should not interpret n_jobs=None as explicitely set because it used to mean 'unset'. Instead we should treat it identically as if it were left to its default value.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.06 ⚠️

Comparison is base (476ff8e) 94.94% compared to head (b70321f) 94.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1475      +/-   ##
==========================================
- Coverage   94.94%   94.88%   -0.06%     
==========================================
  Files          45       45              
  Lines        7474     7491      +17     
==========================================
+ Hits         7096     7108      +12     
- Misses        378      383       +5     

Impacted Files	Coverage Δ
joblib/parallel.py	96.91% <100.00%> (+0.01%)	⬆️
joblib/test/test_config.py	97.95% <100.00%> (+0.36%)	⬆️

... and 3 files with indirect coverage changes

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[tomMoral]

Indeed, thanks for the PR.

[jeremiedbb]

It's not fully fixed yet, i'm still working out some details

[tomMoral]

Yes it is not fixed, it seems like we need to manage and test when n_job is not set on the context and None is passed explicitly

[tomMoral]

Better fix indeed! thanks @jeremiedbb 😁

just an extra nom regression test.

[jeremiedbb]

Actually, n_jobs=None should mean "unset" only for Parallel. For parallel_backend, since the default is -1, None used to mean "set" and was used to reset n_jobs to the backend default n_jobs in a nesting situation. parallel_config should behave the same.

[tomMoral]

LGTM, thx @jeremiedbb

I was quite surprised by the behavior of setting None in the context but indeed, if this is what was enforced, I think it is a good idea to keep the same behavior.