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

[prateek9623]

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• 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.
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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

[STeveShary]

@prateek9623 What is the curl command that you get from using the SwaggerUI?

[prateek9623]

@STeveShary

[STeveShary]

@prateek9623 Ok, the problem is that when you use Swagger, it is creating a HTTP GET request where you can only accept application/json content. That is why you are getting your error. The swagger UI is creating a request that will only accept JSON.

You are not declaring what type of response the request can get, so swagger is setting it as application/json by default. I would suggest that you look at: https://fastapi.tiangolo.com/advanced/custom-response/ to set the correct response object.

This should modify the swagger UI to send the accept header of the correct encoding.

[prateek9623]

@STeveShary ok, but my issue is not with response, my issue is request header, it is not sent correctly. It should be sending image/png but it is sending application/json which i am assuming is default behaviour for "Accept" header. What i think is happening hear is, this default behaviour is overwriting user given "Accept" header.

[STeveShary]

You are correct that the request header is incorrect. This can be corrected by defining the response type.

[prateek9623]

@STeveShary Hi, as per your suggestion I have assigned "StreamingResponse" as respone_class. But still "Accept" header is not user-given, now it is "/".

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

app = FastAPI()


class Message(BaseModel):
    message: str

@app.get("/image", response_class= StreamingResponse, responses={"406": {"model":Message}})
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)
            },
        )

[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:
swagger-api/swagger-ui#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

[prateek9623]

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