FIX: short-circuit n_jobs=None to n_jobs=1 without Parallel setup

doc/changes/dev/13777.bugfix.rst

@@ -0,0 +1 @@
+Avoid some unnecessary computations when ``n_jobs=None`` is equivalent to ``n_jobs=1``, by `Simon Kern`_.

mne/parallel.py

@@ -86,7 +86,7 @@ def parallel_func(
             if n_jobs is not None:
                 warn("joblib not installed. Cannot run in parallel.")
             n_jobs = 1
-    if n_jobs == 1:
+    if (n_jobs == 1) or (n_jobs is None and not _running_in_joblib_context()):
         n_jobs = 1
         my_func = func
         parallel = list
@@ -154,6 +154,16 @@ def parallel_progress(op_iter):
     return parallel_out, my_func, n_jobs
 
 
+def _running_in_joblib_context():
+    """Check if we are running in a joblib.parallel_config context manager."""
+    try:
+        from joblib.parallel import get_active_backend
+    except ImportError:
+        return False
+    _, n_jobs = get_active_backend()
+    return n_jobs is not None
+
+
 def _check_n_jobs(n_jobs):
     n_jobs = _ensure_int(n_jobs, "n_jobs", must_be="an int or None")
     if os.getenv("MNE_FORCE_SERIAL", "").lower() in ("true", "1") and n_jobs != 1:

mne/tests/test_parallel.py

@@ -4,6 +4,7 @@
 
 import multiprocessing
 import os
+import sys
 from contextlib import nullcontext
 
 import pytest
@@ -50,3 +51,29 @@ def fun(x):
     with ctx:
         parallel, p_fun, got_jobs = parallel_func(fun, n_jobs, verbose="debug")
     assert got_jobs == want_jobs
+
+
+def test_parallel_func_n_jobs_none():
+    """Test n_jobs=None is same as n_jobs=1."""
+    joblib = pytest.importorskip("joblib")
+
+    def fun(x):
+        return x * 2
+
+    # test that n_jobs=None (outside context) behaves identically to n_jobs=1.
+    parallel_none, p_fun_none, n_jobs_none = parallel_func(fun, n_jobs=None)
+    parallel_one, p_fun_one, n_jobs_one = parallel_func(fun, n_jobs=1)
+
+    assert parallel_none is parallel_one is list
+    assert n_jobs_none == n_jobs_one == 1
+    assert p_fun_none is p_fun_one is fun, "fun should not be wrapped but is"
+
+    # TODO: test does not work on windows somehow, fix
+    if sys.platform != "win32":
+        # Test that n_jobs=None inside a joblib context uses Parallel.
+        with joblib.parallel_config(backend="loky", n_jobs=2):
+            parallel, p_fun, n_jobs = parallel_func(fun, n_jobs=None)
+
+        assert n_jobs == 2
+        assert parallel is not list
+        assert fun is not p_fun, "fun should be wrapped but is not"