Raise error with unexpected enum value

Author: jkimbo

graphql/execution/executor.py

@@ -437,10 +437,15 @@ def complete_leaf_value(return_type, result):
     """
     Complete a Scalar or Enum by serializing to a valid value, returning null if serialization is not possible.
     """
-    # serialize = getattr(return_type, 'serialize', None)
-    # assert serialize, 'Missing serialize method on type'
+    assert hasattr(return_type, 'serialize'), 'Missing serialize method on type'
+    serialized_result = return_type.serialize(result)
 
-    return return_type.serialize(result)
+    if serialized_result is None:
+        raise GraphQLError(
+            ('Expected a value of type "{}" but ' +
+             'received: {}').format(return_type, result)
+        )
+    return serialized_result
 
 
 def complete_abstract_value(exe_context, return_type, field_asts, info, result):

graphql/type/tests/test_enum_type.py

@@ -127,11 +127,11 @@ def test_does_not_accept_values_with_incorrect_casing():
                                        'Expected type "Color", found green.'
 
 
-# TODO
 def test_does_not_accept_incorrect_internal_value():
     result = graphql(Schema, '{ colorEnum(fromString: "GREEN") }')
     assert result.data == {'colorEnum': None}
-    assert result.errors[0].messages == ''
+    assert result.errors[0].message == 'Expected a value of type "Color" ' \
+                                       'but received: GREEN'
 
 
 def test_does_not_accept_internal_value_in_place_of_enum_literal():