Not getting user user given value for "Accept" header from swagger ui.

First Check

• I added a very descriptive title to this issue.
• I used the GitHub search to find a similar issue and didn’t find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google “How to X in FastAPI” and didn’t find any information.
• I already read and followed all the tutorial in the docs and didn’t find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

from fastapi import FastAPI, Header
from typing import Optional
import cv2
import io
import numpy as np
from fastapi.responses import StreamingResponse, JSONResponse

app = FastAPI()

@app.get("/image")
def get_image(accept: Optional[str] = Header("image/png")):
    accept_headers = accept.split(",")
    dummy = np.zeros((512, 512))
    if "image/png" in accept_headers or "*/*" in accept_headers:
        return StreamingResponse(
            io.BytesIO(cv2.imencode(".png", dummy)[1]), media_type="image/png"
        )
    elif "image/jpeg" in accept_headers:
        return StreamingResponse(
            io.BytesIO(cv2.imencode(".jpeg", dummy)[1]), media_type="image/jpeg"
        )
    else:
        return JSONResponse(
            status_code=406,
            content={
                "message": "encoding of type {} is not supported".format(accept)
            },
        )

Description

Hi, I have an API that is sending responses in different encoding depending on the user and to detect which encoding server should use I am using the “Accept” header. But when I tried to test it using Swagger UI, it always sent “application/json”.

Operating System

Linux, Windows

Operating System Details

No response

FastAPI Version

0.70.1

Python Version

Python 3.9.5

Additional Context

No response

[prateek9623]

i already knew this api work from source other than swagger, but it would have been nice if it also worked on swagger.

[rafsaf]

Observations:

1. In this code, accept is overwritten by openapi docs as you observed:

   @app.get("/image")
   def get_image(accept: str = Header("image/png")):
       return StreamingResponse(
           io.BytesIO("test".encode()), media_type=accept
       )
   

   BUT if in terminal you type

   curl -X 'GET' \
     'http://localhost:8000/image' \
     -H 'accept: image/png'
   

   It works just fine for me.

2. If then we change accept to custom_accept:

   @app.get("/image")
   def get_image(custom_accept: str = Header("image/png")):
       return StreamingResponse(
           io.BytesIO("test".encode()), media_type=custom_accept
       )
   

   You get working custom_accept header just as expected and docs are not broken.

Conclusions:

It’s not FastAPI related, not in any direct way I mean. I found something that may be interesting for you: https://github.com/swagger-api/swagger-ui/issues/7374

That’s kind of expected for me that your accept header will be overwritten by default one.

So, with any luck, you have 3 options:

• either use custom header, in the end I think it’s acceptable solution
• or read https://fastapi.tiangolo.com/advanced/extending-openapi/, read mentioned issue above and possible solution, extend/manipulate your OpenAPI schema and it should work.
• if you don’t care about generated OpenAPI docs, just leave it as is, your endpoint will work, but docs will stay a bit not clean