Extract completeLeafValue from CompleteValue

Related GraphQL-js commit: graphql/graphql-js@4c5904c

Author: Sam Cooke

graphql/execution/executor.py

@@ -255,12 +255,7 @@ def complete_value(self, ctx, return_type, field_asts, info, result):
 
         # If field type is Scalar or Enum, serialize to a valid value, returning null if coercion is not possible.
         if isinstance(return_type, (GraphQLScalarType, GraphQLEnumType)):
-            serialized_result = return_type.serialize(result)
-
-            if serialized_result is None:
-                return None
-
-            return serialized_result
+            return self.complete_leaf_value(ctx, return_type, field_asts, info, result)
 
         runtime_type = None
 
@@ -318,6 +313,17 @@ def complete_list_value(self, ctx, return_type, field_asts, info, result):
 
         return DeferredList(completed_results) if contains_deferred else completed_results
 
+    def complete_leaf_value(self, ctx, return_type, field_asts, info, result):
+        """
+        Complete a Scalar or Enum by serializing to a valid value, returning null if serialization is not possible.
+        """
+        serialized_result = return_type.serialize(result)
+
+        if serialized_result is None:
+            return None
+
+        return serialized_result
+
     def resolve_or_error(self, resolve_fn, source, args, info):
         curried_resolve_fn = functools.partial(resolve_fn, source, args, info)