fastapi with custom classes in response

simple.py

from fastapi import FastAPI
from pydantic import BaseModel, BaseConfig

app = FastAPI()
BaseConfig.arbitrary_types_allowed = True


class Foo:
    def __init__(self, x: int):
        self.x = x


class Response(BaseModel):
    foo: Foo


@app.get("/", response_model=Response)
def root():
    return Response(foo=Foo(1))


if __name__ == '__main__':
    import uvicorn
    uvicorn.run("main:app")
    # route works and returns {"foo":{"x":1}}, OpenAPI crashes:
    # ValueError: Value not declarable with JSON Schema,
    # field: name='foo' type=Foo required=True