type check error on add_exception_handler()

[feng000000]

When I use add_exception_handler() to register a custom exception handler, Pylance raise a warning "Exception" is not assignable to "ValidationException"

My code:

from starlette.applications import Starlette
from starlette.exceptions import HTTPException
from starlette.requests import Request
from starlette.responses import JSONResponse

app = Starlette()

class ValidationException(HTTPException):
    ...

# I prefer not to set the type hint of `e` to `Exception` because I want to retain the IntelliSense for `ValidationException`
def validation_exception_handler(req: Request, e: ValidationException):
    return JSONResponse({})

app.add_exception_handler(ValidationException, validation_exception_handler)

Should we consider using TypeVar to resolve the type checking issue, or am I using it incorrectly?

---

Possible change

• starlette/types.py

# ...
ExceptionType = typing.TypeVar("ExceptionType")
HTTPExceptionHandler = typing.Callable[["Request", ExceptionType], "Response | typing.Awaitable[Response]"]
WebSocketExceptionHandler = typing.Callable[["WebSocket", ExceptionType], typing.Awaitable[None]]
ExceptionHandler = typing.Union[HTTPExceptionHandler[ExceptionType], WebSocketExceptionHandler[ExceptionType]]

# before:
HTTPExceptionHandler = typing.Callable[["Request", Exception], "Response | typing.Awaitable[Response]"]
WebSocketExceptionHandler = typing.Callable[["WebSocket", Exception], typing.Awaitable[None]]
ExceptionHandler = typing.Union[HTTPExceptionHandler, WebSocketExceptionHandler]

• starlette/applications.py

ExceptionType = typing.TypeVar("ExceptionType")
class Starlette:
    # ...
    def add_exception_handler(
        self,
        exc_class_or_status_code: int | type[ExceptionType],
        handler: ExceptionHandler[ExceptionType],
    ) -> None:  # pragma: no cover
        self.exception_handlers[exc_class_or_status_code] = handler
    # ...
    
    
# before
class Starlette:
    # ...
    def add_exception_handler(
        self,
        exc_class_or_status_code: int | type[Exception],
        handler: ExceptionHandler,
    ) -> None:  # pragma: no cover
        self.exception_handlers[exc_class_or_status_code] = handler
    # ...

after change:

[Kludex]

• Check Generic ExceptionHandler type #2403