fix: allow resolve_type to return a type name string for Interface and Union

graphene/types/schema.py

@@ -397,6 +397,9 @@ def resolve_type(self, resolve_type_func, type_name, root, info, _type):
         if inspect.isclass(type_) and issubclass(type_, ObjectType):
             return type_._meta.name
 
+        if isinstance(type_, str):
+            return type_
+
         return_type = self[type_name]
         return default_type_resolver(root, info, return_type)
 

graphene/types/tests/test_interface.py

@@ -202,3 +202,42 @@ def resolve_test(_, info):
     )
     assert not result.errors
     assert result.data == {"test": {"__typename": "MyTestType1", "type": "foo"}}
+
+
+def test_resolve_type_custom_string():
+    class MyInterface(Interface):
+        field2 = String()
+
+        @classmethod
+        def resolve_type(cls, instance, info):
+            if instance["type"] == 1:
+                return "MyTestType1"
+            return "MyTestType2"
+
+    class MyTestType1(ObjectType):
+        class Meta:
+            interfaces = (MyInterface,)
+
+    class MyTestType2(ObjectType):
+        class Meta:
+            interfaces = (MyInterface,)
+
+    class Query(ObjectType):
+        test = Field(MyInterface)
+
+        def resolve_test(_, info):
+            return {"type": 1}
+
+    schema = Schema(query=Query, types=[MyTestType1, MyTestType2])
+
+    result = schema.execute(
+        """
+        query {
+            test {
+                __typename
+            }
+        }
+    """
+    )
+    assert not result.errors
+    assert result.data == {"test": {"__typename": "MyTestType1"}}