[QUESTION] Response model with pydantic's arbitrary_types_allowed not working

I use pydantic model with custom type. Set in config arbitrary_types_allowed=True. Without setting a parameter “response_model” it’s work. But when I set this model in param:

fastapi.exceptions.FastAPIError: Invalid args for response field! Hint: check that <class 'Foo'> is a valid pydantic field type 

Pydantic class definition example:

import Foo

class Model(BaseModel):
    id: str
    custom: Optional[Foo] = Field(None)
    status: str = None
    fail_message: Optional[str] = None
    params: Dict[str, Any] = None

    @validator('custom')
    def deserialize_model(cls, v):
        return Foo.loads(v)

    class Config:
        arbitrary_types_allowed = True
        json_encoders = {Foo: lambda v: v.dumps()}

[fubupc]

Same problem here:

from pydantic import BaseModel
from fastapi import FastAPI


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


class M(BaseModel):
    pair: Pair
    other: int

    class Config:
        arbitrary_types_allowed = True


app = FastAPI()


@app.get("/m/", response_model=M)
def just_m():
    return M(pair=Pair(x=5, y=7), other=8)

will report error:

fastapi.exceptions.FastAPIError: Invalid args for response field! Hint: check that <class 'test.Pair'> is a valid pydantic field type

[bcb]

I ended up using a custom Pydantic field validator, instead of using arbitrary_types_allowed:

class AssociationList(_AssociationList):
    @classmethod
    def __get_validators__(cls):
        yield cls.validate

    @classmethod
    def validate(cls, v):
        if not isinstance(v, _AssociationList):
            raise TypeError("AssociationList required")
        return [UserOut.from_orm(u) for u in v]


class TeamOut(TeamBase):
    members: AssociationList
    ...