from typing import Union

from fastapi import FastAPI
from fastapi.testclient import TestClient
from pydantic import BaseModel, ConfigDict


class FooBaseModel(BaseModel):
    model_config = ConfigDict(extra="forbid")


class Foo(FooBaseModel):
    pass


app = FastAPI()


@app.post("/")
async def post(
    foo: Union[Foo, None] = None,
):
    return foo


client = TestClient(app)