Custom response does not include headers set in dependencies

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Example Code

import fastapi


def set_custom_header(response: fastapi.Response) -> None:
    response.headers["X-Custom-Header"] = "Testing Response Dependencies"


app = fastapi.FastAPI(dependencies=[fastapi.Depends(set_custom_header)])


@app.get("/response")
def response():
    return {"status": "ok"}


@app.get("/custom_response")
def custom_response():
    return fastapi.responses.JSONResponse({"status": "ok"})

Description

When an endpoint returns an explicit Response headers set in dependencies are not set. However, if one returns the plain data and relies on the default response then the headers are included.

With the example code above, I one fetches /response then the header x-custom-header will be set, but not when using the endpoint /custom_response.

$ curl localhost:8000/response -v
> GET /response HTTP/1.1
> Host: localhost:8000
> User-Agent: curl/7.64.1
> Accept: */*
>
< HTTP/1.1 200 OK
< date: Sat, 15 Oct 2022 19:09:55 GMT
< server: uvicorn
< content-length: 15
< x-custom-header: Testing Responde dependencies
<
{"status":"ok"}


$ curl localhost:8000/custom_response -v
> GET /custom_response HTTP/1.1
> Host: localhost:8000
> User-Agent: curl/7.64.1
> Accept: */*
>
< HTTP/1.1 200 OK
< date: Sat, 15 Oct 2022 19:09:58 GMT
< server: uvicorn
< content-length: 15
<
{"status":"ok"}

The part where the headers are added to the default response is here

https://github.com/tiangolo/fastapi/blob/90fc4299d1d24a54dda8bf6f01cf3779ce6cf467/fastapi/routing.py#L263-L264

and here is the code where a custom response is returned

https://github.com/tiangolo/fastapi/blob/90fc4299d1d24a54dda8bf6f01cf3779ce6cf467/fastapi/routing.py#L235-L238

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.79.0

Python Version

Python 3.10.8

Additional Context

No response

[jgould22]

This feels like expected behavior.

You have edited the response object in the dependency and then created a fresh new response in your controller without the edited headers.

Anything you have done to the response object previously is thrown away in favor of your new response object.

If you wanted to be sure all responses get a header and not worry about the controller you could do it with an ASGI middle-ware which would add the header after the controller is run.

[tiangolo]

Yep, as @jgould22 says, returning responses directly is a scape hatch for you to override anything FastAPI would do and take over directly: https://fastapi.tiangolo.com/advanced/response-directly/#return-a-response

They are not intended as the main use case. And yes, this means there are two ways to do things, do them through the normal ways, returning data directly, or taking over the wheel and doing things yourself. It’s not great to have two ways to do things, but there’s no other way to let you take the wheel, so we have to leave with that.

Maybe that section in the docs could include another note apart from saying that any data validation won’t be done, and also say that no additional headers or background tasks set in other places will be included. If you think that would make things clearer for you, please make a PR there to the docs with that extra note. 🙏