response_model ignores @property getter

[charlax]

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Example

Here's a self-contained, minimal, reproducible, example with my use case:

from typing import Optional

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

app = FastAPI()


class UserIn(BaseModel):
    username: str

    @property
    def email(self):
        return f"{self.username}@foo.com"


class UserOut(BaseModel):
    username: str
    email: str


@app.get("/users/1", response_model=UserOut)
async def get_user():
    return UserIn(username="toaster")



client = TestClient(app)


def test_get():
    response = client.get("/users/1")
    assert response.status_code == 200
    assert response.json() == {"email": "toaster@gdc.com"}

Description

• Run with pytest
• Watch the test fail
• But I expected it to pass

Failure:

  File "testfastapi/env/lib/python3.9/site-packages/fastapi/routing.py", line 209, in app
    response_data = await serialize_response(
  File "testfastapi/env/lib/python3.9/site-packages/fastapi/routing.py", line 126, in serialize_response
    raise ValidationError(errors, field.type_)
pydantic.error_wrappers.ValidationError: 1 validation error for UserOut
response -> email
  field required (type=value_error.missing)

Environment

• OS: [e.g. Linux / Windows / macOS]: macOS
• FastAPI Version [e.g. 0.3.0]: 0.63.0
• Python version: 3.9.1

Additional context

This is probably related to pydantic/pydantic#935 - but I'm wondering if we shouldn't use something like from_orm when creating response object, because nothing guarantees (I believe?) that the returned object will be a BaseModel?

In any case, thanks a lot for your work on both libraries!

[Mause]

The easiest way would be to set orm_mode on UserOut

[charlax]

The easiest way would be to set orm_mode on UserOut

Tried it, but as expected the fastapi code would have to do from_orm for this to work as well...

[Mause]

Yes, which it will? Did you get another error?

[charlax]

Yes, same error...

from typing import Optional

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

app = FastAPI()


class UserIn(BaseModel):
    username: str

    @property
    def email(self):
        return f"{self.username}@gdc.com"


class UserOut(BaseModel):
    username: str
    email: str

    class Config:
        orm_mode = True


@app.get("/users/1", response_model=UserOut)
async def get_user():
    return UserIn(username="toaster")



client = TestClient(app)


def test_get():
    response = client.get("/users/1")
    assert response.status_code == 200
    assert response.json() == {"email": "toaster@gdc.com"}

I could be wrong but you need (1) orm_mode = True and (2) from_orm(...) (which requires change in serialize_response).

[Mause]

Nope, it looks like it's not working because fastapi under the hood calls .dict() on the model, which means the later orm_mode functionality won't see the field. You could override this method, or alternatively, do something like this:

class UserIn(BaseModel):
    username: str
    email: Optional[EmailStr] = None
    
    def __init__(self, username, email=None):
        super().__init__(username=username, email=email)
        self.email = f"{self.username}@gdc.com"

[charlax]

Marshmallow allows using a property... I haven't had time to finish reading the code, but this essentially restricts the kind of objects that can be returned from a router function... Using simple getattr() instead of calling .dict() would allow for using both Pydantic models AND any other object, including plain objects/ORM objects/property on BaseModel, a net benefit - what do you think?

[Mause]

I mean, this isn't a limitation of FastAPI at all, it's a limitation of pydantic. Ultimately if you wish to use marshmallow instead for your internal models, there's nothing stopping you

[charlax]

I'd love to keep everything under the fastapi/pydantic umbrella - I mentioned marshmallow because this shows that it seems an intuitive feature to support...

It's true that some part of this feature should live within pydantic. Do you recommend I open a ticket there too?

[PrettyWood]

Hi @charlax!
It is definitely related to pydantic! You're welcome to try with pydantic/pydantic#2625 in local to see if this suits your needs :)