[BUG] Unable to specify schema for specific media type if same as response_class

Describe the bug

When the (inferred) response_class matches the media type key in responses, I can’t override the schema for that media type—it just uses the response_model value. The media type schema override does work for a media type that doesn’t match response_class.

To Reproduce

Steps to reproduce the behavior with a minimum self-contained file.

Replace each part with your own scenario:

1. Create a file with:

from typing import Union, List
from enum import Enum

from fastapi import FastAPI, Header
from starlette import status
from pydantic import BaseModel, Field

app = FastAPI()


class DogsJson(BaseModel):
    dogs: list


class DogFeature(BaseModel):
    type: str = Field("Feature", const=True)
    geometry: dict
    properties: dict


class DogsGeoJson(BaseModel):
    features: List[DogFeature]


class DogsRequestAcceptHeaders(Enum):
    json = "application/json"
    geojson = "application/geo+json"


@app.post(
    "/dogs",
    response_model=Union[DogsJson, DogsGeoJson],
    responses={
        status.HTTP_200_OK: {
            "content": {
                DogsRequestAcceptHeaders.json.value: {
                    "schema": {
                        "$ref": f"#/components/schemas/{DogsJson.__name__}"
                    }
                },
                DogsRequestAcceptHeaders.geojson.value: {
                    "schema": {
                        "$ref": f"#/components/schemas/{DogsGeoJson.__name__}"
                    }
                },
            }
        },
    },
)
async def get_dogs(
        accept: DogsRequestAcceptHeaders = Header(DogsRequestAcceptHeaders.json),
) -> Union[DogsJson, DogsGeoJson]:
    dogs = ["dog_1", "dog_2"]

    if accept == DogsRequestAcceptHeaders.json:
        return DogsJson(dogs=dogs)
    else:
        dog_features = [DogFeature(geometry={}, properties={"name": dog}) for dog in dogs]
        return DogsGeoJson(features=dog_features)


from uvicorn import run
run(app, host="localhost", port=8000)

1. Open the browser at localhost:8000/docs.
2. Inspect the 200 response for /dogs and note that media type application/json’s schema is still derived from response_model and not overridden by what’s in responses. Also note that application/geo+json is overridden as expected.

Expected behavior

OpenAPI schema declarations in responses should override any other conflicting definitions.

Screenshots

application/json schema matches response_model when it should match what’s in responses.

application/geo+json schema matches what’s in responses, as expected.

Environment

• OS: macOS
• FastAPI Version: 0.48.0
• Python version: 3.8.1

[tiangolo]

Thanks for the help here everyone! 👏 🙇

Thanks for reporting back and closing the issue @maflaven 👍

[maflaven]

@eth3lbert yes this is fixed!