Raise exception in python-fastApi middleware

Question

I am trying to validate token in fastapi middleware but this seems impossible. As i am thinking middleware needs to make next call although its not required. I am not able to find any good solution to handle token in one go in this python-fastapi backend. Any help is appreciated.

@app.middleware("http")
async def add_middleware_here(request: Request, call_next):
    token = request.headers["Authorization"]
    try:
        verification_of_token = verify_token(token)
        if verification_of_token:
            response = await call_next(request)
            return response
    except InvalidSignatureError as er:
        raise HTTPException(status_code=401)

Answer 1

You need to return a response. I'll show you how you can make it work:

from fastapi.responses import JSONResponse

@app.middleware("http")
async def add_middleware_here(request: Request, call_next):
    token = request.headers["Authorization"]
    try:
        verification_of_token = verify_token(token)
        if verification_of_token:
            response = await call_next(request)
            return response
        else:
            return JSONResponse(status_code=403) # or 401
    except InvalidSignatureError as er:
        return JSONResponse(status_code=401)

Be aware that using this middleware implies that you don't have a login route on your API (for token generation).

Besides, you should consider using this dependency instead: https://fastapi.tiangolo.com/tutorial/security/simple-oauth2/

Another thing is that you can use fastapi.status for your status code.

Answer 2

See JSONResponse API documentation.

from fastapi.responses import JSONResponse

@app.middleware("http")
async def errors_handling(request: Request, call_next):
    try:
        return await call_next(request)
    except Exception as exc:
        return JSONResponse(status_code=500, content={'reason': str(exc)})