Union types cannot be resovled. Documentation incomplete

I am setting up a GraphQL Server with Python using Starlette and Graphene and ran into a problem I cannot find a solution for. The Graphene Documentation does not go into detail regarding the union type, which I am trying to implement. I set up a minimum example based on the graphene documentation which you can run to replicate this problem

import os

import uvicorn
from graphene import ObjectType, Field, List, String, Int, Union
from graphene import Schema
from starlette.applications import Starlette
from starlette.graphql import GraphQLApp
from starlette.routing import Route

mock_data = {
    "episode": 3,
    "characters": [
        {
            "type": "Droid",
            "name": "R2-D2",
            "primaryFunction": "Astromech"
        },
        {
            "type": "Human",
            "name": "Luke Skywalker",
            "homePlanet": "Tatooine"
        },
        {
            "type": "Starship",
            "name": "Millennium Falcon",
            "length": 35
        }
    ]
}


class Human(ObjectType):
    name = String()
    homePlanet = String()


class Droid(ObjectType):
    name = String()
    primary_function = String()


class Starship(ObjectType):
    name = String()
    length = Int()


class Characters(Union):
    class Meta:
        types = (Human, Droid, Starship)


class SearchResult(ObjectType):
    characters = List(Characters)
    episode = Int()


class RootQuery(ObjectType):
    result = Field(SearchResult)

    @staticmethod
    def resolve_result(_, info):
        return mock_data


graphql_app = GraphQLApp(schema=Schema(query=RootQuery))

routes = [
    Route("/graphql", graphql_app),
]

api = Starlette(routes=routes)

if __name__ == "__main__":
    uvicorn.run(api, host="127.0.0.1", port=int(os.environ.get("PORT", 8080)))

If you then go to http://localhost:8080/graphq and enter the following query

query Humans{
  result {
    episode
    characters {
      ... on Human {
        name
      }
    }
  }
}
I get this error

 {
    "data": {
        "result": {
            "episode": 3,
            "characters": null
        }
    },
    "errors": [
        {
            "message": "Abstract type Characters must resolve to an Object type at runtime for field SearchResult.characters with value \"[{'type': 'Droid', 'name': 'R2-D2', 'primaryFunction': 'Astromech'}, {'type': 'Human', 'name': 'Luke Skywalker', 'homePlanet': 'Tatooine'}, {'type': 'Starship', 'name': 'Millennium Falcon', 'length': 35}]\", received \"None\".",
            "locations": [
                {
                    "line": 4,
                    "column": 5
                }
            ]
        }
    ]
}

which I am now stuck with. Maybe someone has done this already and can help out? How can I resolve this at runtime. I have already tried different approaches for example I changed classes Character and RootQuery:

class Character(Union):
    class Meta:
        types = (Human, Droid, Starship)

    def __init__(self, data, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.data = data
        self.type = data.get("type")

    def resolve_type(self, info):
        if self.type == "Human":
            return Human
        if self.type == "Droid":
            return Droid
        if self.type == "Starship":
            return Starship


class RootQuery(ObjectType):
    result = Field(SearchResult)
    
    @staticmethod
    def resolve_result(_, info):
        return {**mock_data, "characters": [Character(character) for character in mock_data.get('characters')]}

resulting in

{
    "data": {
        "result": {
            "episode": 3,
            "characters": [
                {},
                {
                    "name": null
                },
                {}
            ]
        }
    }
}

Any ideas would be very appreciated!

[R1nat-P]

@jkimbo this works. Thanks a lot.

How it worked? It returns only 1 class attributes because in resolve_type is return to the 1 class, and not all attributes from union-ed classes.

Thanks!

[R1nat-P]

@R1nat-P it does not have anything to do with the union-ed class. instance is the actual data with which the class is called and instance[“type”] is just the attribute. You could also do with instance.get(“type”). I suggest you to have a closer look at the source code or even set the example up your self locally. Btw, here you can find the stackoverflow article corresponding to it. Hope this helps.

@jmandt ,

Actually was done like was described in official documentation link , therefore was added the part with resolve_type , no luck.

class LoginData(graphene.Union):
    class Meta:
        types = (UsersData, MembershipData)

    @classmethod
    def resolve_type(cls, instance, info):
            if instance["type"] == "UsersData":
                return UsersData
            if instance["type"] == "MembershipData":
                return MembershipData

And getting the following result

Thanks!