Custom Handler

In the event that you are using a third party library with a custom error class, a handler specifically a common base class can be provided.

Providing a custom handler allows for conversion from the custom error class into a Problem, when the exception handler catches it, rather than converting each raised instance into a Problem at the time it is raised.

Usage

Given a third_party library with a error.py module.

class CustomBaseError(Exception):
    def __init__(reason: str, debug: str):
        self.reason = reason
        self.debug = debug

A custom handler can then be defined in your application.

import fastapi
from rfc9457 import error_class_to_type
from fastapi_problem.error import Problem
from fastapi_problem.handler import ExceptionHandler, add_exception_handler
from starlette.requests import Request

from third_party.error import CustomBaseError

def my_custom_handler(eh: ExceptionHandler, request: Request, exc: CustomBaseError) -> Problem:
    return Problem(
        title=exc.reason,
        detail=exc.debug,
        type_=error_class_to_type(exc),
        status=500,
        headers={"x-custom-header": "value"},
    )

app = fastapi.FastAPI()
add_exception_handler(
    app,
    handlers={
        CustomBaseError: my_custom_handler,
    },
)

Any instance of CustomBaseError, or any subclasses, that reach the exception handler will then be converted into a Problem response, as opposed to an unhandled error response.

Optional handling

In some cases you may want to handle specific cases for a type of exception, but let others defer to another handler. In these scenarios, a custom handler can return None rather than a Problem. If a handler returns None the exception will be pass to the next defined handler.

import fastapi
from rfc9457 import error_class_to_type
from fastapi_problem.error import Problem
from fastapi_problem.handler import ExceptionHandler, add_exception_handler
from starlette.requests import Request

def no_response_handler(eh: ExceptionHandler, request: Request, exc: RuntimeError) -> Problem | None:
    if str(exc) == "No response returned.":
        return Problem(
            title="No response returned.",
            detail="starlette bug",
            type_="no-response",
            status=409,
        )
    return None

def base_handler(eh: ExceptionHandler, request: Request, exc: Exception) -> Problem:
    return Problem(
        title=exc.reason,
        detail=exc.debug,
        type_=error_class_to_type(exc),
        status=500,
    )

app = fastapi.FastAPI()
add_exception_handler(
    app,
    handlers={
        RuntimeError: no_response_handler,
        Exception: base_handler,
    },
)

At the time of writing there was (is?) a bug in starlette that would cause middlewares to error. To prevent these from reaching Sentry, a deferred handler was implemented in the impacted project.