[RFC] Additional optional arguments

Closes #26

Author: jhgg

graphql/core/type/schema.py

@@ -159,9 +159,12 @@ def assert_object_implements_interface(object, interface):
         for object_arg in object_field.args:
             arg_name = object_arg.name
             interface_arg = interface_arg_map.get(arg_name)
-            assert interface_arg, (
-                '{}.{} does not define argument "{}" but {}.{} provides it.'
-            ).format(interface, field_name, arg_name, object, field_name)
+            if not interface_arg:
+                assert not isinstance(object_arg.type, GraphQLNonNull), (
+                    '{}.{}({}:) is of required type '
+                    '"{}" but is not also provided by the '
+                    'interface {}.{}.'
+                ).format(object, field_name, arg_name, object_arg.type, interface, field_name)
 
 
 def is_equal_type(type_a, type_b):

tests/core_type/test_validation.py

@@ -1288,7 +1288,7 @@ def test_accepts_an_object_which_implements_an_interface_along_with_more_fields(
 
         assert schema_with_field_type(AnotherObject)
 
-    def test_rejects_an_object_which_implements_an_interface_field_along_with_more_arguments(self):
+    def test_accepts_an_object_which_implements_an_interface_field_along_with_more_arguments(self):
         AnotherInterface = GraphQLInterfaceType(
             name='AnotherInterface',
             resolve_type=_none,
@@ -1316,11 +1316,41 @@ def test_rejects_an_object_which_implements_an_interface_field_along_with_more_a
             }
         )
 
+        assert schema_with_field_type(AnotherObject)
+
+    def test_rejects_an_object_which_implements_an_interface_field_along_with_additional_required_arguments(self):
+        AnotherInterface = GraphQLInterfaceType(
+            name='AnotherInterface',
+            resolve_type=_none,
+            fields={
+                'field': GraphQLField(
+                    type=GraphQLString,
+                    args={
+                        'input': GraphQLArgument(GraphQLString)
+                    }
+                )
+            }
+        )
+
+        AnotherObject = GraphQLObjectType(
+            name='AnotherObject',
+            interfaces=[AnotherInterface],
+            fields={
+                'field': GraphQLField(
+                    type=GraphQLString,
+                    args={
+                        'input': GraphQLArgument(GraphQLString),
+                        'anotherInput': GraphQLArgument(GraphQLNonNull(GraphQLString)),
+                    }
+                ),
+            }
+        )
+
         with raises(AssertionError) as excinfo:
             schema_with_field_type(AnotherObject)
 
-        assert str(excinfo.value) == 'AnotherInterface.field does not define argument "anotherInput" but ' \
-                                     'AnotherObject.field provides it.'
+        assert str(excinfo.value) == 'AnotherObject.field(anotherInput:) is of required type "String!" but ' \
+                                     'is not also provided by the interface AnotherInterface.field.'
 
     def test_rejects_an_object_missing_an_interface_field(self):
         AnotherInterface = GraphQLInterfaceType(