enh: expand test coverage

nitransforms/resampling.py

@@ -156,8 +156,7 @@ def apply(
             else None
         )
 
-        if njobs is None:
-            njobs = cpu_count()
+        njobs = cpu_count() if njobs is None or njobs < 1 else njobs
 
         with ProcessPoolExecutor(max_workers=min(njobs, n_resamplings)) as executor:
             results = []

nitransforms/tests/test_resampling.py

@@ -15,7 +15,7 @@
 from nitransforms import nonlinear as nitnl
 from nitransforms import manip as nitm
 from nitransforms import io
-from nitransforms.resampling import apply
+from nitransforms.resampling import apply, _apply_volume
 
 RMSE_TOL_LINEAR = 0.09
 RMSE_TOL_NONLINEAR = 0.05
@@ -363,3 +363,16 @@ def test_LinearTransformsMapping_apply(
             reference=testdata_path / "sbref.nii.gz",
             serialize_nvols=2 if serialize_4d else np.inf,
         )
+
+
+@pytest.mark.parametrize("t", list(range(4)))
+def test_apply_helper(monkeypatch, t):
+    """Ensure the apply helper function correctly just decorates with index."""
+    from nitransforms.resampling import ndi
+
+    def _retval(*args, **kwargs):
+        return 1
+
+    monkeypatch.setattr(ndi, "map_coordinates", _retval)
+
+    assert _apply_volume(t, None, None) == (t, 1)