FastAPI==0.89.0 Cannot use None as a return type when status_code is set to 204 with from __future__ import annotations

[matiuszka]

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Example Code

from __future__ import annotations 

from fastapi import FastAPI

app = FastAPI()


@app.get("/", status_code=204)
def read_root() -> None:
    return {"Hello": "World"}

Description

If we add:
from __future__ import annotations

It changes the annotations structure and the response model is NoneType instead of None, which causes validation of the statuc_code vs response_model and raises an exception.

  ...
  File ".../site-packages/fastapi/routing.py", line 635, in decorator
    self.add_api_route(
  File ".../site-packages/fastapi/routing.py", line 574, in add_api_route
    route = route_class(
  File ".../site-packages/fastapi/routing.py", line 398, in __init__
    assert is_body_allowed_for_status_code(
AssertionError: Status code 204 must not have a response body

I am working on a fix for it right now.

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.89.0

Python Version

3.10

Additional Context

No response

[tiangolo]

Thanks for the report! ☕

This is because the return type is being used to generate the response model, it's a new "feature" 😅

You can solve it with response_model=None.

There's a lot of new docs about it here: https://fastapi.tiangolo.com/tutorial/response-model/#other-return-type-annotations

from __future__ import annotations 

from fastapi import FastAPI

app = FastAPI()


@app.get("/", status_code=204, response_model=None)
def read_root() -> None:
    return {"Hello": "World"}

[matiuszka]

@tiangolo thanks, it helped :)

Although I think this is not cool if there is an error or not depending on if you will add from __future__ import annotations or not.

Are there any plans to improve it? I did proposal here how it can be solved: #5860