FIX n_jobs=None behavior in Parallel and config contexts (#1475)

CHANGES.rst

@@ -1,6 +1,13 @@
 Latest changes
 ==============
 
+Release 1.3.2 -- In developpement
+---------------------------------
+
+- Fix a regression in ``joblib.Parallel`` introduced in 1.3.0 where
+  explicitly setting ``n_jobs=None`` was not interpreted as "unset".
+  https://github.com/joblib/joblib/pull/1475
+
 Release 1.3.1 -- 2023/06/29
 ---------------------------
 

joblib/parallel.py

@@ -1188,6 +1188,10 @@ def __init__(
         prefer=default_parallel_config["prefer"],
         require=default_parallel_config["require"],
     ):
+        # Interpret n_jobs=None as 'unset'
+        if n_jobs is None:
+            n_jobs = default_parallel_config["n_jobs"]
+
         active_backend, context_config = _get_active_backend(
             prefer=prefer, require=require, verbose=verbose
         )

joblib/test/test_config.py

@@ -123,3 +123,29 @@ def test_threadpool_limitation_in_child_context_error(context, backend):
 
     with raises(AssertionError, match=r"does not acc.*inner_max_num_threads"):
         context(backend, inner_max_num_threads=1)
+
+
+@parametrize("context", [parallel_config, parallel_backend])
+def test_parallel_n_jobs_none(context):
+    # Check that n_jobs=None is interpreted as "unset" in Parallel
+    # non regression test for #1473
+    with context(backend="threading", n_jobs=2):
+        with Parallel(n_jobs=None) as p:
+            assert p.n_jobs == 2
+
+    with context(backend="threading"):
+        default_n_jobs = Parallel().n_jobs
+        with Parallel(n_jobs=None) as p:
+            assert p.n_jobs == default_n_jobs
+
+
+@parametrize("context", [parallel_config, parallel_backend])
+def test_parallel_config_n_jobs_none(context):
+    # Check that n_jobs=None is interpreted as "explicitly set" in
+    # parallel_(config/backend)
+    # non regression test for #1473
+    with context(backend="threading", n_jobs=2):
+        with context(backend="threading", n_jobs=None):
+            # n_jobs=None resets n_jobs to backend's default
+            with Parallel() as p:
+                assert p.n_jobs == 1