Customize SanicException response body

error_handling.py

from sanic import response, Sanic
from sanic.exceptions import SanicException
from sanic.request import Request
from sanic.response import HTTPResponse


# NOTE: This error should only be raised after add_custom_error_handler()
#       has been called to register a custom error handler on the
#       currently-running app.
class CustomSanicException(SanicException):
    def __init__(self, *args, body: object, **kwargs) -> None:
        super().__init__(*args, **kwargs)
        
        # (At first glance it seems like `self.message` could be used as the
        # `body` argument to `response.json()`, but it seems that
        # `Exception.__init__()` converts `message` to a string before assigning
        # it to `self.message`.)
        # 
        # (It also seems like the `message` argument to `__init__()` could be
        # assigned to `self.body`, but SanicException (and maybe also Exception)
        # have that argument type-annotated with an incompatible type that
        # doesn't allow `message` to be a dict, which is commonly what we want
        # the body to be.)
        self.body = body


# Adds an error handler that catches CustomSanicException errors and responds
# with a JSON body containing the value of `exception.body`.
def add_custom_error_handler(app: Sanic) -> None:
    @app.exception(CustomSanicException)
    async def handle_custom_sanic_exception(
            request: Request, exception: CustomSanicException) -> HTTPResponse:
        
        return response.json(
            status=exception.status_code, body=exception.body)