Typing error for app.add_exception_handler (Argument "handler" to "add_exception_handler" of "Starlette" has incompatible type)

[aryaniyaps]

I have my custom errors which are subclasses of the Exception class itself

Here's an example:

# app/core/errors.py
class BaseError(Exception):
    """Base error class."""

    def __init__(self, message: str) -> None:
        self.message = message
        super().__init__(self.message)


class InvalidInputError(BaseError):
    """Indicate that the client has issued an invalid request."""

and this is my corresponding error handler:

# app/core/error_handlers.py
from fastapi import Request
from .errors import InvalidInputError

async def handle_invalid_input_error(
    _request: Request,
    exception: InvalidInputError,
) -> Response:
    """Handle InvalidInputError expections."""
    return ORJSONResponse(
        content={
            "message": exception.message,
        },
        status_code=status.HTTP_400_BAD_REQUEST,
    )

I am using FastAPI but I believe that the error arises from the add_exception_handler method defined in starlette.

While initializing my app like this:

# app/__init__.py
from fastapi import FastAPI

from app.core.error_handlers import handle_invalid_input_error
from app.core.errors import InvalidInputError


def create_app() -> FastAPI:
    """Initialize an app instance."""
    app = FastAPI(version="0.0.1")
    app.add_exception_handler(
        InvalidInputError,
        handler=handle_invalid_input_error,
    )
    return app

I get this mypy error:

error: Argument "handler" to "add_exception_handler" of "Starlette" has incompatible type "Callable[[Request, InvalidInputError], Coroutine[Any, Any, Response]]"; expected
"Callable[[Request, Exception], Response | Awaitable[Response]] | Callable[[WebSocket, Exception], Awaitable[None]]"  [arg-type]
            handler=handle_invalid_input_error,

Is this a potential issue in the typing of the method?

[HenriBlacksmith]

I face the same issue, the changes in typing made in this PR: #2180 seem to cause this as typing is too strict.

I assume TypeVar bound to Exception and Response would be the proper type hints instead of Exception and Response

Current typing:

HTTPExceptionHandler = typing.Callable[
    ["Request", Exception], typing.Union["Response", typing.Awaitable["Response"]]
]

R = typing.TypeVar("R", bound="Response")
E = typing.TypeVar("E", bound="Exception")

HTTPExceptionHandler = typing.Callable[
    ["Request", E], typing.Union[R, typing.Awaitable[R]]
]

Edit: Actually it seems that a
PR is already opened to fix this: #2403