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

[zadigus]

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Example Code

import logging

from fastapi import APIRouter
from fastapi.responses import JSONResponse

liveness = True

router = APIRouter()


@router.get(
    "/livez",
    summary="Liveness probe",
    description="Returns 200 if the service is alive",
)
async def get_liveness() -> JSONResponse:
    logging.debug("GET /livez")
    if liveness:
        return JSONResponse(status_code=200, content={"status": "ok"})
    else:
        return JSONResponse(status_code=500, content={"status": "not ok"})

Description

With version 0.88.0, my unit tests run fine, I get no error. With version 0.89.0, I get the following error:

/root/.pyenv/versions/3.8/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
<frozen importlib._bootstrap>:1014: in _gcd_import
    ???
<frozen importlib._bootstrap>:991: in _find_and_load
    ???
<frozen importlib._bootstrap>:975: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:671: in _load_unlocked
    ???
/root/.pyenv/versions/3.8/lib/python3.8/site-packages/_pytest/assertion/rewrite.py:168: in exec_module
    exec(co, module.__dict__)
tests/models/conftest.py:7: in <module>
    from my_sample_lib.main import app
my_sample_lib/main.py:10: in <module>
    from my_sample_lib.health.router import router as health_router
my_sample_lib/health/router.py:24: in <module>
    async def get_liveness() -> JSONResponse:
/root/.pyenv/versions/3.8/lib/python3.8/site-packages/fastapi/routing.py:633: in decorator
    self.add_api_route(
/root/.pyenv/versions/3.8/lib/python3.8/site-packages/fastapi/routing.py:572: in add_api_route
    route = route_class(
/root/.pyenv/versions/3.8/lib/python3.8/site-packages/fastapi/routing.py:400: in __init__
    self.response_field = create_response_field(
/root/.pyenv/versions/3.8/lib/python3.8/site-packages/fastapi/utils.py:90: in create_response_field
    raise fastapi.exceptions.FastAPIError(
E   fastapi.exceptions.FastAPIError: Invalid args for response field! Hint: check that <class 'starlette.responses.JSONResponse'> is a valid pydantic field type

The only way I found to get rid of the error is to remove the type hint, which I don't really want. Also, I tried to follow the guidelines provided in your documentation without success.

Interestingly, the whole thing works fine (with the type hint and fastapi 0.89.0) under Windows 10. Under Ubuntu 22.10, it doesn't work at all (i.e. it provides the above message).

Operating System

Linux

Operating System Details

Under Windows 10, it works like a charm. Under Ubuntu 22.10, it fails with the provided error above.

FastAPI Version

0.89.0

Python Version

3.8.12

Additional Context

It is also failing with python 3.9, and 3.10.

[ThirVondukr]

I encountered the same error with response classes

@zadigus You can specify response_model manually but I'm not sure if that's correct here 🤔

[moadennagi]

pydantic does not recognize JSONReponse as being a valid field type, pydantic documentation says to add arbitrary_types_allowed in the Config to allow for arbitrary user types: (https://docs.pydantic.dev/usage/model_config/)

whether to allow arbitrary user types for fields (they are validated simply by checking if the value is an instance of the type). If False, RuntimeError will be raised on model declaration (default: False). See an example in Field Types.

The issue happens in fastapi.utils.create_response_field which calls pydantic.fields.ModelField (partially), it passes down model_config=pydantic.config.BaseConfig as a default value.
Perhaps fastapi needs to add a way to pass custom config class when constructing the APIRoute to be passed down to create_response_field

class MyConfig:
    arbitrary_types_allowed = True

[ThirVondukr]

pydantic does not recognize JSONReponse as being a valid field type, pydantic documentation says to add arbitrary_types_allowed in the Config to allow for arbitrary user types: (https://docs.pydantic.dev/usage/model_config/)

whether to allow arbitrary user types for fields (they are validated simply by checking if the value is an instance of the type). If False, RuntimeError will be raised on model declaration (default: False). See an example in Field Types.

The issue happens in fastapi.utils.create_response_field which calls pydantic.fields.ModelField (partially), it passes down model_config=pydantic.config.BaseConfig as a default value. Perhaps fastapi needs to add a way to pass custom config class when constructing the APIRoute to be passed down to create_response_field

class MyConfig:
    arbitrary_types_allowed = True

I think adding arbitrary_types_allowed=True would only cause Response to be interpreted as

content: Any
status_code: int
headers: ...
...

(As in Response.__init__), and what would be preferred is either fastapi recognizing Response subclasses or mentioning it in documentation and recommending a workaround (for example passing response_model=None explicitly)

[novitae]

Same issue here, it is only checking for instances of pydantic.fields.ModelField, and ignoring instances of starlette.responses.Response, where comes from JSONResponse ...

It is also failing with python 3.9, and 3.10.

And 3.11

[rassie]

Pretty sure it's a regression in release 0.89, since its only new feature deals with response_models: #1436.

EDIT: Forgot to mention: my code worked with 0.88, but doesn't with 0.89, which is why I'm sure it's the culprit.

[iudeen]

Looks like a regression. Try with 0.88 if the issue exist?

[rassie]

Yes, 0.88 works as expected.

[hofrob]

Small script to reproduce:

• python 3.11
• FastAPI 0.89

from fastapi import FastAPI, Response


app = FastAPI(debug=True)


@app.get("/plain-text")
def plain_text() -> Response:
    return Response(content="bla", media_type="text/plain")

Running this with python main.py will show this error.

Traceback (most recent call last):
  File "../fastapi-regression/main.py", line 7, in <module>
    @app.get("/plain-text")
     ^^^^^^^^^^^^^^^^^^^^^^
  File "../fastapi-regression/venv/lib/python3.11/site-packages/fastapi/routing.py", line 633, in decorator
    self.add_api_route(
  File "../fastapi-regression/venv/lib/python3.11/site-packages/fastapi/routing.py", line 572, in add_api_route
    route = route_class(
            ^^^^^^^^^^^^
  File "../fastapi-regression/venv/lib/python3.11/site-packages/fastapi/routing.py", line 400, in __init__
    self.response_field = create_response_field(
                          ^^^^^^^^^^^^^^^^^^^^^^
  File "../fastapi-regression/venv/lib/python3.11/site-packages/fastapi/utils.py", line 90, in create_response_field
    raise fastapi.exceptions.FastAPIError(
fastapi.exceptions.FastAPIError: Invalid args for response field! Hint: check that <class 'starlette.responses.Response'> is a valid pydantic field type

[rassie]

@hofrob with a test case so small, you'd have to wonder why the tests haven't catched this before release.

[ThirVondukr]

This issue should be fixed by #5855

[hofrob]

@hofrob with a test case so small, you'd have to wonder why the tests haven't catched this before release.

I guess we're all invited to add tests and fixes to this project. 🤷

[moadennagi]

This issue should be fixed by #5855

The pr sets response_model=None for Response subclasses, I wonder if there is way to support subclasses of Response

[tiangolo]

Thanks for the report and discussion everyone! ☕

The simple use case of a single Response class is now supported in FastAPI 0.89.1 (just release) 🎉

And for the other use cases where you want to have more complex type annotations (e.g. unions, other types), you can use response_model=None.

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

Also, this is sort of a duplicate of #5857 (it's the same issue underneath).

[zadigus]

Fastapi version 0.89.1 solves the issue in my codebase.

[github-actions]

Assuming the original need was handled, this will be automatically closed now. But feel free to add more comments or create new issues or PRs.

[lendoo73]

I have the same issue
To resolve this issue I found only one way:

1. pip show fastapi: Version: 0.101.0
2. pip uninstall fastapi
3. Then I modified my requirements.txt from fastapi to fastapi==0.88
4. pip install -r requirements.txt
5. pip uninstall fastapi
6. Then I modified my requirements.txt again from fastapi==0.88 to fastapi
7. pip install -r requirements.txt

And now my server runs without error...

[CharlesPerrotMinotHCHB]

I think we should open a new issue, but I am also reproducing, using fastapi 0.103.1, and migrating from pydantic v1 to pydantic v2 (2.4.1, tried with 2.2.1 as well)

[Kludex]

I think we should open a new issue, but I am also reproducing, using fastapi 0.103.1, and migrating from pydantic v1 to pydantic v2 (2.4.1, tried with 2.2.1 as well)

Please create a discussion. This issue was solved.