BUG: Fix bug with not short-circuiting n_jobs=1 (#13147)

mne/parallel.py

@@ -128,6 +128,21 @@ def run_verbose(*args, verbose=logger.level, **kwargs):
 
         my_func = delayed(run_verbose)
 
+        # if we got that n_jobs=1, we shouldn't bother with any parallelization
+        if n_jobs == 1:
+            # TODO: Hack until https://github.com/joblib/joblib/issues/1687 lands
+            try:
+                backend_repr = str(parallel._backend)
+            except Exception:
+                backend_repr = ""
+            is_local = any(
+                f"{x}Backend" in backend_repr
+                for x in ("Loky", "Threading", "Multiprocessing")
+            )
+            if is_local:
+                my_func = func
+                parallel = list
+
     if total is not None:
 
         def parallel_progress(op_iter):

mne/tests/test_filter.py

@@ -418,14 +418,21 @@ def test_resample_scipy():
 
 
 @pytest.mark.parametrize("n_jobs", (2, "cuda"))
-def test_n_jobs(n_jobs):
+def test_n_jobs(n_jobs, capsys):
     """Test resampling against SciPy."""
+    joblib = pytest.importorskip("joblib")
     x = np.random.RandomState(0).randn(4, 100)
     y1 = resample(x, 2, 1, n_jobs=None)
     y2 = resample(x, 2, 1, n_jobs=n_jobs)
     assert_allclose(y1, y2)
-    y1 = filter_data(x, 100.0, 0, 40, n_jobs=None)
-    y2 = filter_data(x, 100.0, 0, 40, n_jobs=n_jobs)
+    capsys.readouterr()
+    with joblib.parallel_config(backend="loky", n_jobs=1):
+        y1 = filter_data(x, 100.0, 0, 40, n_jobs=None, verbose=True)
+    out, err = capsys.readouterr()
+    # if it's in there, we didn't triage properly
+    assert "Parallel(" not in out
+    assert "Parallel(" not in err
+    y2 = filter_data(x, 100.0, 0, 40, n_jobs=n_jobs, verbose=True)
     assert_allclose(y1, y2)