Support inheritance of __typename attribute (#25)

src/graphql/execution/execute.py

@@ -1092,6 +1092,18 @@ def invalid_return_type_error(
     )
 
 
+def get_typename(value: Any) -> Optional[str]:
+    """Get the `__typename` property of the given value."""
+    if isinstance(value, dict):
+        return value.get("__typename")
+    # need to de-mangle the attribute assumed to be "private" in Python
+    for cls in value.__class__.__mro__:
+        __typename = getattr(value, f"_{cls.__name__}__typename", None)
+        if __typename:
+            return __typename
+    return None
+
+
 def default_type_resolver(
     value: Any, info: GraphQLResolveInfo, abstract_type: GraphQLAbstractType
 ) -> AwaitableOrValue[Optional[Union[GraphQLObjectType, str]]]:
@@ -1107,12 +1119,7 @@ def default_type_resolver(
     for the object being coerced, returning the first type that matches.
     """
     # First, look for `__typename`.
-    type_name = (
-        value.get("__typename")
-        if isinstance(value, dict)
-        # need to de-mangle the attribute assumed to be "private" in Python
-        else getattr(value, f"_{value.__class__.__name__}__typename", None)
-    )
+    type_name = get_typename(value)
     if isinstance(type_name, str):
         return type_name
 

tests/utilities/test_build_ast_schema.py

@@ -517,6 +517,46 @@ class RootValue:
                 == expected
             )
 
+        def using_inheritance():
+            class Fruit:
+                __typename = "Fruit"
+
+            class Apple(Fruit):
+                __typename = "Apple"
+
+            class Delicious(Apple):
+                color = "golden or red"
+
+            class GoldenDelicious(Delicious):
+                color = "golden"
+
+            class RedDelicious(Delicious):
+                color = "red"
+
+            class GrannySmith(Apple):
+                color = "green"
+
+            class Banana(Fruit):
+                __typename = "Banana"
+                length = 5
+
+            class RootValue:
+                fruits = [GrannySmith(), RedDelicious(), GoldenDelicious(), Banana()]
+
+            assert graphql_sync(
+                schema=schema, source=source, root_value=RootValue()
+            ) == (
+                {
+                    "fruits": [
+                        {"color": "green"},
+                        {"color": "red"},
+                        {"color": "golden"},
+                        {"length": 5},
+                    ]
+                },
+                None,
+            )
+
     def describe_specifying_interface_type_using_typename():
         schema = build_schema(
             """
@@ -600,6 +640,38 @@ class RootValue:
                 == expected
             )
 
+        def using_inheritance():
+            class Character:
+                __typename = "Character"
+
+            class Human(Character):
+                __typename = "Human"
+
+            class HanSolo(Human):
+                name = "Han Solo"
+                totalCredits = 10
+
+            class Droid(Character):
+                __typename = "Droid"
+
+            class RemoteControlled:
+                name = "R2"
+
+            class Mobile:
+                name = "D2"
+
+            class R2D2(RemoteControlled, Droid, Mobile):
+                name = "R2-D2"
+                primaryFunction = "Astromech"
+
+            class RootValue:
+                characters = [HanSolo(), R2D2()]
+
+            assert (
+                graphql_sync(schema=schema, source=source, root_value=RootValue())
+                == expected
+            )
+
     def custom_scalar():
         sdl = dedent(
             """