Resolver function for union type in Ariadne

I am trying to write a query resolver function for union type in Ariadne. How can I accomplish this?

As I have read in the documentation there is a field called __typename which helps us to resolve the union type. But I am not getting any __typename to my resolver function.

Schema

type User {
  username: String!
  firstname: String
  email: String
}

type UserDuplicate {
  username: String!
  firstname: String
  email: String
}

union UnionTest = User | UserDuplicate

type UnionForCustomTypes {
  user: UnionTest
  name: String!
}

type Query {
  user: String!
  unionForCustomTypes: [UnionForCustomTypes]!
}

Ariadne resolver functions

query = QueryType()
mutation = MutationType()
unionTest = UnionType("UnionTest")


@unionTest.type_resolver
def resolve_union_type(obj, *_):
    if obj[0]["__typename"] == "User":
        return "User"
    if obj[0]["__typename"] == "DuplicateUser":
        return "DuplicateUser"

    return None


# Query resolvers
@query.field("unionForCustomTypes")
def resolve_union_for_custom_types(_, info):
    result = [
        {"name": "Manisha Bayya", "user": [{"__typename": "User", "username": "abcd"}]}
    ]   
    return result

Query I am trying

{
  unionForCustomTypes {
    name
    user {
      __typename
      ...on User {
        username
        firstname
      }
    }
  }
}

When I try the query I am getting below error

{
  "data": null,
  "errors": [
    {
      "message": "Cannot return null for non-nullable field Query.unionForCustomTypes.",
      "locations": [
        [
          2,
          3
        ]
      ],
      "path": [
        "unionForCustomTypes"
      ],
      "extensions": {
        "exception": {
          "stacktrace": [
            "Traceback (most recent call last):",
            "  File \"/root/manisha/prisma/ariadne_envs/lib/python3.6/site-packages/graphql/execution/execute.py\", line 675, in complete_value_catching_error",
            "    return_type, field_nodes, info, path, result",
            "  File \"/root/manisha/prisma/ariadne_envs/lib/python3.6/site-packages/graphql/execution/execute.py\", line 754, in complete_value",
            "    \"Cannot return null for non-nullable field\"",
            "TypeError: Cannot return null for non-nullable field Query.unionForCustomTypes."
          ],
          "context": {
            "completed": "None",
            "result": "None",
            "path": "ResponsePath(...rCustomTypes')",
            "info": "GraphQLResolv...f04e9c1fc50>})",
            "field_nodes": "[FieldNode at 4:135]",
            "return_type": "<GraphQLNonNu...ustomTypes'>>>",
            "self": "<graphql.exec...x7f04e75677f0>"
          }
        }
      }
    }
  ]
}

We don't need any resolver for union type. We can just send __typename field while returning the owner field. In my code, I am returning list for owner attribute which is wrong. I have to just send a dictionary.

Below is the change I have done in my code to make it work.

# Deleted resolver for UnionType

@query.field("unionForCustomTypes")
def resolve_union_for_custom_types(_, info):
    result = [{"name": "Manisha Bayya", "user": {"__typename": "User", "username": "abcd", "firstname": "pqrs"}}] # <-- Line changed

    return result