Add DefaultValue.disallow (#1546)

traits-stubs/traits-stubs/constants.pyi

@@ -64,6 +64,8 @@ class DefaultValue(IntEnum):
     callable_and_args: int = ...
     callable: int = ...
     trait_set_object: int = ...
+    disallow: int = ...
+
 
 MAXIMUM_DEFAULT_VALUE_TYPE: Any
 default_value_map: Any

traits/constants.py

@@ -197,6 +197,10 @@ class DefaultValue(IntEnum):
     #: is the default value.
     trait_set_object = traits.ctraits._TRAIT_SET_OBJECT_DEFAULT_VALUE
 
+    #: This trait doesn't permit a default value, and an attempt to retrieve
+    #: the default value using the default_value_for method will fail.
+    disallow = traits.ctraits._DISALLOW_DEFAULT_VALUE
+
 
 #: Maximum legal value for default_value_type, for use in testing
 #: and validation.
@@ -215,4 +219,5 @@ class DefaultValue(IntEnum):
     DefaultValue.callable_and_args: "factory",
     DefaultValue.callable: "method",
     DefaultValue.trait_set_object: "set",
+    DefaultValue.disallow: "invalid",
 }

traits/ctrait.py

@@ -60,6 +60,7 @@ def default(self):
             DefaultValue.object,
             DefaultValue.callable_and_args,
             DefaultValue.callable,
+            DefaultValue.disallow,
         ):
             return Undefined
         elif kind in (

traits/ctraits.c

@@ -186,9 +186,12 @@ call_notifiers(
    is the default value */
 #define TRAIT_SET_OBJECT_DEFAULT_VALUE 9
 
+/* This trait doesn't permit a default value */
+#define DISALLOW_DEFAULT_VALUE 10
+
 /* Maximum legal value for default_value_type, for use in testing and
    validation. */
-#define MAXIMUM_DEFAULT_VALUE_TYPE 9
+#define MAXIMUM_DEFAULT_VALUE_TYPE 10
 
 /* The maximum value for comparison_mode. Valid values are between 0 and
    the maximum value. */
@@ -1845,6 +1848,12 @@ default_value_for(trait_object *trait, has_traits_object *obj, PyObject *name)
         case TRAIT_SET_OBJECT_DEFAULT_VALUE:
             return call_class(
                 TraitSetObject, trait, obj, name, trait->default_value);
+        case DISALLOW_DEFAULT_VALUE:
+            PyErr_SetString(
+                PyExc_ValueError,
+                "default value not permitted for this trait"
+            );
+            return NULL;
     }
     return result;
 }
@@ -5726,6 +5735,14 @@ PyInit_ctraits(void)
     if (error < 0) {
         return NULL;
     }
+    error = PyModule_AddIntConstant(
+        module,
+        "_DISALLOW_DEFAULT_VALUE",
+        DISALLOW_DEFAULT_VALUE
+    );
+    if (error < 0) {
+        return NULL;
+    }
     error = PyModule_AddIntConstant(
         module,
         "_MAXIMUM_DEFAULT_VALUE_TYPE",

traits/tests/test_trait_types.py

@@ -110,6 +110,28 @@ class MyTraitType(TraitType):
             (DefaultValue.constant, Undefined),
         )
 
+    def test_disallowed_default_value(self):
+        class MyTraitType(TraitType):
+
+            default_value_type = DefaultValue.disallow
+
+        trait_type = MyTraitType()
+        self.assertEqual(
+            trait_type.get_default_value(),
+            (DefaultValue.disallow, Undefined)
+        )
+
+        ctrait = trait_type.as_ctrait()
+        self.assertEqual(
+            ctrait.default_value(),
+            (DefaultValue.disallow, Undefined),
+        )
+        self.assertEqual(ctrait.default_kind, "invalid")
+        self.assertEqual(ctrait.default, Undefined)
+
+        with self.assertRaises(ValueError):
+            ctrait.default_value_for(None, "<dummy>")
+
 
 class TestDeprecatedTraitTypes(unittest.TestCase):
     def test_function_deprecated(self):