[BUG] Union doesn't document properly on swagger

Describe the bug

When using Union on response_model, it doesn’t doucment properly on swagger.

To Reproduce

1. Create two models

from pydantic import BaseModel

class VerifyTokenResponseSchema(BaseModel):
    status: bool

class ExceptionResponseSchema(BaseModel):
    error: str

1. Add it on response_model

from typing import Union
from fastapi import APIRouter

oauth_router = APIRouter()

@oauth_router.post('/verify', response_model=Union[VerifyTokenResponseSchema, ExceptionResponseSchema])
async def verify_token(token: str):
    status = await VerifyTokenUsecase().execute(token=token)
    return status.dict()

1. Check it on /docs

Expected behavior

Example value tab throw no example available.

But it can show on Schema tab

Additional context

I want to check both response schema on Example value tab.

Is there anything wrong with my code?

(I know that I can use responses but using Union is described on document(https://fastapi.tiangolo.com/tutorial/extra-models/#union-or-anyof)

[tiangolo]

Thanks for the help here @Dustyposa and @sm-Fifteen ! 👏 🍰

Yep, indeed that’s not a bug in FastAPI, but a feature request for Swagger UI.

Nevertheless, you can add a custom example that is shown in Swagger UI, check the docs: https://fastapi.tiangolo.com/tutorial/schema-extra-example/

[phy25]

@mcauto Please see comments above. It’s not supported by Swagger UI so you have to add a custom example. Also -

What do you want to display in example value like Union type?

from fastapi import FastAPI, Body
from typing import Union
from pydantic import BaseModel


class User(BaseModel):
    name: str


class Item(BaseModel):
    size: int
    price: float


app = FastAPI()

process_things_body_example = {"name": "1", "size": 2, "price": 3} # whatever makes sense to you

@app.post("/multi/")
def process_things(body: Union[User, Item] = Body(..., example=process_things_body_example)):
    return body

Please create an issue at https://github.com/swagger-api/swagger-ui if you want this behavior to be improved.