"AttributeError: __pydantic_model__" when response_model is union

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Example Code

import typing

import fastapi
import httpx
import pydantic as pydantic
import pytest
from fastapi import FastAPI
from fastapi.encoders import jsonable_encoder
from pydantic import ValidationError
from starlette import status
from starlette.responses import JSONResponse


class ResponseOne(pydantic.BaseModel):
    val: str


class ResponseTwo(pydantic.BaseModel):
    val: str


app = FastAPI()
incorrect_response = {"abc": "def"}


@app.exception_handler(ValidationError)
async def validation_exception_handler(request: fastapi.Request, exc: ValidationError):
    return JSONResponse(
        status_code=status.HTTP_400_BAD_REQUEST,
        content=jsonable_encoder({"detail": exc.errors()}),
    )


@app.get("/union", response_model=typing.Union[ResponseOne, ResponseTwo])
async def union():
    return incorrect_response


@app.get("/single", response_model=ResponseOne)
async def single():
    return incorrect_response


@pytest.fixture
async def client():
    async with httpx.AsyncClient(app=app, base_url="http://localhost") as client:
        yield client


async def test_unprintable(client):
    res = await client.get("/union")
    assert res.status_code == 400


async def test_400(client):
    res = await client.get("/single")
    assert res.status_code == 400

Description

When response_model is set to a union of two possible models and I return something that does not match one of the two models, FastAPI returns a 500 error:

test_.py:30: in validation_exception_handler
    content=jsonable_encoder({"detail": exc.errors()}),
../../../../../Library/Caches/pypoetry/virtualenvs/bla/lib/python3.9/site-packages/pydantic/error_wrappers.py:63: in errors
    config = self.model.__pydantic_model__.__config__  # type: ignore
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = typing.Union[tests.test_.ResponseOne, tests.test_.ResponseTwo]
attr = '__pydantic_model__'

    def __getattr__(self, attr):
        # We are careful for copy and pickle.
        # Also for simplicity we just don't relay all dunder names
        if '__origin__' in self.__dict__ and not _is_dunder(attr):
            return getattr(self.__origin__, attr)
>       raise AttributeError(attr)
E       AttributeError: __pydantic_model__

../../../../../.pyenv/versions/3.9.9/lib/python3.9/typing.py:710: AttributeError

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.81.0

Python Version

3.9.9

Additional Context

It also does not work in FastAPI 0.7* and the pydantic version (1.9 vs 1.0) has no influence, either.

[chbndrhnns]

Here is a test with pure pydantic which shows the behaviour.

ValidationError seems not to support a union of models for the model parameter.

def test_error_from_union():
    class One(BaseModel):
        val: int

    class Two(BaseModel):
        val: int

    with pytest.raises(ValidationError) as err_one:
        One()

    with pytest.raises(ValidationError) as err_two:
        Two(val="abc")

    model = typing.Union[One, Two]

    error = ValidationError([err_one, err_two], model=model)
    assert error.errors()

[chbndrhnns]

@JarroVGIT thanks for bringing it up. Yes, it is.