How do I integrate custom exception handling with the FastAPI exception handling?

Python version 3.9, FastAPI version 0.78.0

I have a custom function that I use for application exception handling. When requests run into internal logic problems, i.e I want to send an HTTP response of 400 for some reason, I call a utility function.

@staticmethod
def raise_error(error: str, code: int) -> None:
    logger.error(error)
    raise HTTPException(status_code=code, detail=error)

Not a fan of this approach. So I look at

from fastapi import FastAPI, HTTPException, status
from fastapi.respones import JSONResponse

class ExceptionCustom(HTTPException):
    pass


def exception_404_handler(request: Request, exc: HTTPException):
    return JSONResponse(status_code=status.HTTP_404_NOT_FOUND, content={"message": "404"})


app.add_exception_handler(ExceptionCustom, exception_404_handler)

The problem I run into with the above approach is the inability to pass in the message as an argument.

Any thoughts on the whole topic?

Solution

Your custom exception can have any custom attributes that you want. Let's say you write it this way:

class ExceptionCustom(HTTPException):
    pass

in your custom handler, you can do something like

def exception_404_handler(request: Request, exc: HTTPException):
    return JSONResponse(status_code=status.HTTP_404_NOT_FOUND, content={"message": exc.detail})

Then, all you need to do is to raise the exception this way:

raise ExceptionCustom(status_code=404, detail='error message')

Note that you are creating a handler for this specific ExceptionCustom. If all you need is the message, you can write something more generic:

class MyHTTPException(HTTPException):
    pass

def my_http_exception_handler(request: Request, exc: HTTPException):
    return JSONResponse(status_code=exc.status_code, content={"message": exc.detail})

app.add_exception_handler(MyHTTPException, my_http_exception_handler)

This way you can raise any exception, with any status code and any message and have the message in your JSON response.

There's a detailed explanation on FastAPI docs