Fix dynamic default values for Union traits

traits/tests/test_union.py

@@ -10,8 +10,9 @@
 
 import unittest
 
-from traits.api import (Float, Instance, Int, Str, TraitError, TraitType,
-                        HasTraits, Union, Type)
+from traits.api import (
+    Float, Instance, Int, List, Str, TraitError, TraitType, HasTraits, Union,
+    Type)
 
 
 class CustomClass(HasTraits):
@@ -27,7 +28,7 @@ def validate(self, obj, name, value):
         self.error(obj, name, value)
 
 
-class TestCaseEnumTrait(unittest.TestCase):
+class TestUnion(unittest.TestCase):
 
     def test_union_incompatible_trait(self):
         with self.assertRaises(ValueError) as exception_context:
@@ -179,6 +180,25 @@ class TestClass(HasTraits):
 
         TestClass(s="sdf")
 
+    def test_list_inside_union_default(self):
+        class HasUnionWithList(HasTraits):
+            foo = Union(List(Int), Str)
+
+        has_union = HasUnionWithList()
+        value = has_union.foo
+        self.assertIsInstance(value, list)
+        with self.assertRaises(TraitError):
+            value.append("not an integer")
+
+    def test_constant_default(self):
+        # Exercise the branch where the default is constant.
+        class HasUnionWithList(HasTraits):
+            foo = Union(Int(23), Float)
+
+        has_union = HasUnionWithList()
+        value = has_union.foo
+        self.assertEqual(value, 23)
+
 
 if __name__ == '__main__':
     unittest.main()

traits/trait_types.py

@@ -43,7 +43,6 @@
 from .trait_list_object import TraitListEvent, TraitListObject
 from .trait_set_object import TraitSetEvent, TraitSetObject
 from .trait_type import (
-    _infer_default_value_type,
     NoDefaultSpecified,
     TraitType,
 )
@@ -4001,15 +4000,25 @@ def __init__(self, *traits, **metadata):
                 "'default'."
             )
 
-        default_value = None
         if 'default_value' in metadata:
             default_value = metadata.pop("default_value")
-        elif self.list_ctrait_instances:
-            default_value = self.list_ctrait_instances[0].default
+        else:
+            first_default_value, first_default_value_type = (
+                self.list_ctrait_instances[0].default_value())
+
+            if first_default_value_type == DefaultValue.constant:
+                self.default_value_type = DefaultValue.constant
+                default_value = first_default_value
+            else:
+                self.default_value_type = DefaultValue.callable
+                default_value = self._get_default_value
 
-        self.default_value_type = _infer_default_value_type(default_value)
         super().__init__(default_value, **metadata)
 
+    def _get_default_value(self, object):
+        return self.list_ctrait_instances[0].default_value_for(
+            object, "<inner_trait>")
+
     def validate(self, obj, name, value):
         """ Return the value by the first trait in the list that can
         validate the assigned value, raise an error if none of them can.