raise errors instead now

glue/core/tests/test_visual.py

@@ -1,20 +1,29 @@
 from glue.core.visual import VisualAttributes
 from matplotlib.cm import get_cmap
-
+import pytest
 
 def test_VA_preferred_cmap():
-    # Not a real CMAP array
-    va = VisualAttributes(preferred_cmap=1)
-    assert va.preferred_cmap == 1
+    # Not a real CMAP array - errors
+    with pytest.raises(ValueError,match="`preferred_cmap` must be a string or an instance of a matplotlib.colors.Colormap"):
+        VisualAttributes(preferred_cmap=1)
+
+    # Not a valid string for a CMAP - errors
+    with pytest.raises(ValueError,match="not_a_cmap is not a valid colormap name."):
+        VisualAttributes(preferred_cmap="not_a_cmap")
 
-    # Not a valid string for a CMAP
-    va = VisualAttributes(preferred_cmap="not_a_cmap")
-    assert va.preferred_cmap is None
+    viridis_cmap = get_cmap("viridis")
 
     # get_cmap cmap name
     va = VisualAttributes(preferred_cmap="viridis")
-    assert va.preferred_cmap == get_cmap("viridis")
-
+    assert va.preferred_cmap == viridis_cmap
     # formal cmap name
     va = VisualAttributes(preferred_cmap="Viridis")
-    assert va.preferred_cmap == get_cmap("viridis")
+    assert va.preferred_cmap == viridis_cmap
+
+    # Valid Colormap
+    va = VisualAttributes(preferred_cmap=viridis_cmap)
+    assert va.preferred_cmap == viridis_cmap
+
+    # None is allowed - it is the default
+    va = VisualAttributes(preferred_cmap=None)
+    assert va.preferred_cmap == None

glue/core/visual.py

@@ -1,4 +1,4 @@
-from matplotlib.colors import ColorConverter
+from matplotlib.colors import ColorConverter, Colormap
 from matplotlib.cm import get_cmap
 
 from glue.config import settings
@@ -128,9 +128,11 @@ def preferred_cmap(self, value):
                         break
                 else:
                     # If the string name fails to be validated
-                    self._preferred_cmap = None
-        else:
+                    raise ValueError(f"{value} is not a valid colormap name.")
+        elif isinstance(value, Colormap) or value is None:
             self._preferred_cmap = value
+        else:
+            raise ValueError("`preferred_cmap` must be a string or an instance of a matplotlib.colors.Colormap")
 
     @callback_property
     def alpha(self):