MRG: Better type check for report (#6132)

* FIX: Better type check for report

* FIX: Reword

Author: larsoner

mne/report.py

@@ -69,10 +69,15 @@ def _fig_to_img(fig, image_format='png', scale=None, **kwargs):
         try:
             mlab = _import_mlab()
         # on some systems importing Mayavi raises SystemExit (!)
-        except Exception as e:
-            warn('Could not import mayavi (%r). Trying to render'
-                 '`mayavi.core.api.Scene` figure instances'
-                 ' will throw an error.' % (e,))
+        except Exception:
+            is_mayavi = False
+        else:
+            import mayavi
+            is_mayavi = isinstance(fig, mayavi.core.scene.Scene)
+        if not is_mayavi:
+            raise TypeError('Each fig must be a matplotlib Figure, mayavi '
+                            'Scene, or NumPy ndarray, got %s (type %s)'
+                            % (fig, type(fig)))
         if fig.scene is not None:
             img = mlab.screenshot(figure=fig)
         else:  # Testing mode

mne/tests/test_report.py

@@ -142,6 +142,11 @@ def test_render_report():
     # ndarray support smoke test
     report.add_figs_to_section(np.zeros((2, 3, 3)), 'caption', 'section')
 
+    with pytest.raises(TypeError, match='Each fig must be a'):
+        report.add_figs_to_section('foo', 'caption', 'section')
+    with pytest.raises(TypeError, match='Each fig must be a'):
+        report.add_figs_to_section(['foo'], 'caption', 'section')
+
 
 @testing.requires_testing_data
 def test_report_raw_psd_and_date():