BUG fix reported number of jobs in effective_n_jobs (#985)

CHANGES.rst

@@ -1,6 +1,13 @@
 Latest changes
 ==============
 
+Release 0.14.2
+--------------
+
+- Fix the number of jobs reported by ``effective_n_jobs`` when ``n_jobs=None``
+  called in a parallel backend context.
+  https://github.com/joblib/joblib/pull/985
+
 Release 0.14.1
 --------------
 

continuous_integration/travis/install.sh

@@ -91,6 +91,7 @@ if [ -n "$NUMPY_VERSION" ]; then
 fi
 
 if [[ "$COVERAGE" == "true" ]]; then
+    # TODO: unpin when https://github.com/nedbat/coveragepy/issues/883 is fixed
     # Weird issues with recent version of coverage: unpin when not causing
     # pytest to raise INTERNALERROR exceptions.
     PIP_INSTALL_PACKAGES="$PIP_INSTALL_PACKAGES coverage==4.5.4 pytest-cov codecov"

joblib/parallel.py

@@ -385,7 +385,9 @@ def effective_n_jobs(n_jobs=-1):
     .. versionadded:: 0.10
 
     """
-    backend, _ = get_active_backend()
+    backend, backend_n_jobs = get_active_backend()
+    if n_jobs is None:
+        n_jobs = backend_n_jobs
     return backend.effective_n_jobs(n_jobs=n_jobs)
 
 

joblib/test/test_parallel.py

@@ -152,6 +152,23 @@ def test_effective_n_jobs():
     assert effective_n_jobs() > 0
 
 
+@pytest.mark.parametrize(
+    "backend_n_jobs, expected_n_jobs",
+    [(3, 3), (-1, effective_n_jobs(n_jobs=-1)), (None, 1)],
+    ids=["positive-int", "negative-int", "None"]
+)
+@with_multiprocessing
+def test_effective_n_jobs_None(backend_n_jobs, expected_n_jobs):
+    # check the number of effective jobs when `n_jobs=None`
+    # non-regression test for https://github.com/joblib/joblib/issues/984
+    with parallel_backend("threading", n_jobs=backend_n_jobs):
+        # when using a backend, the default of number jobs will be the one set
+        # in the backend
+        assert effective_n_jobs(n_jobs=None) == expected_n_jobs
+    # without any backend, None will default to a single job
+    assert effective_n_jobs(n_jobs=None) == 1
+
+
 ###############################################################################
 # Test parallel