Fix bug in handling of Union default value (#1534)

* Fix bug in handling of Union default value

* Fix style errors

* Fix spacing bug

traits/tests/test_union.py

@@ -11,8 +11,8 @@
 import unittest
 
 from traits.api import (
-    Float, Instance, Int, List, Str, TraitError, TraitType, HasTraits, Union,
-    Type)
+    Bytes, DefaultValue, Float, HasTraits, Instance, Int, List, Str,
+    TraitError, TraitType, Type, Union)
 
 
 class CustomClass(HasTraits):
@@ -195,10 +195,21 @@ def test_constant_default(self):
         class HasUnionWithList(HasTraits):
             foo = Union(Int(23), Float)
 
+            nested = Union(Union(Str(), Bytes()), Union(Int(), Float(), None))
+
         has_union = HasUnionWithList()
         value = has_union.foo
         self.assertEqual(value, 23)
 
+        self.assertEqual(
+            has_union.trait("foo").default_value(),
+            (DefaultValue.constant, 23),
+        )
+        self.assertEqual(
+            has_union.trait("nested").default_value(),
+            (DefaultValue.constant, ""),
+        )
+
 
 if __name__ == '__main__':
     unittest.main()

traits/trait_types.py

@@ -4028,7 +4028,7 @@ def __init__(self, *traits, **metadata):
         if 'default_value' in metadata:
             default_value = metadata.pop("default_value")
         else:
-            first_default_value, first_default_value_type = (
+            first_default_value_type, first_default_value = (
                 self.list_ctrait_instances[0].default_value())
 
             if first_default_value_type == DefaultValue.constant: