[MAINT] Improve error for nifti masker when fit is called without imgs or mask_img

nilearn/maskers/nifti_masker.py

@@ -428,6 +428,12 @@ def fit(self, imgs=None, y=None):
 
         # Compute the mask if not given by the user
         if self.mask_img is None:
+            if imgs is None:
+                raise ValueError(
+                    "Parameter 'imgs' must be provided to "
+                    f"{self.__class__.__name__}.fit() "
+                    "if no mask is passed to mask_img."
+                )
             mask_args = self.mask_args if self.mask_args is not None else {}
             compute_mask = _get_mask_strategy(self.mask_strategy)
             if self.verbose > 0:

nilearn/maskers/tests/test_nifti_masker.py

@@ -353,18 +353,27 @@ def test_joblib_cache(tmp_path):
         shutil.rmtree(cachedir, ignore_errors=True)
 
 
+def test_fit_no_mask_no_img_error():
+    """Check error is raised when no mask and no img is provided."""
+    mask = NiftiMasker(mask_img=None)
+    with pytest.raises(
+        ValueError, match="Parameter 'imgs' must be provided to "
+    ):
+        mask.fit()
+
+
 def test_mask_strategy_errors(rng):
     """Check that mask_strategy errors are raised."""
     # Error with unknown mask_strategy
+    img = rng.uniform(size=(9, 9, 5))
+    img = nibabel.Nifti1Image(img, np.eye(4))
     mask = NiftiMasker(mask_strategy="oops")
     with pytest.raises(
         ValueError, match="Unknown value of mask_strategy 'oops'"
     ):
-        mask.fit()
+        mask.fit(img)
     # Warning with deprecated 'template' strategy,
     # plus an exception because there's no resulting mask
-    img = rng.uniform(size=(9, 9, 5))
-    img = nibabel.Nifti1Image(img, np.eye(4))
     mask = NiftiMasker(mask_strategy="template")
     with pytest.warns(
         UserWarning, match="Masking strategy 'template' is deprecated."