Type hints for unhandled exceptions code

[dineshbvadhia]

Below is a function to catch unhandled exceptions using excepthook.

import sys
import logging

logger = logging.getLogger(__name__)

# A unhandled exception handler with excepthook
def handle_unhandled_exception(exc_type, exc_value, exc_traceback) -> None:
    if issubclass(exc_type, KeyboardInterrupt):
        # call the default excepthook
        sys.__excepthook__(exc_type, exc_value, exc_traceback)
        return
    # log message
    logger.critical('Unhandled exception:', exc_info=(exc_type, exc_value, exc_traceback))

# assign excepthook to the unhandled exception handler
sys.excepthook = handle_unhandled_exception

Running mypy generates the error:

> unhandled.py:7: error: Function is missing a type annotation for one or more arguments  [no-untyped-def]

What is the correct way to type the traceback parameters in the function? Also, is None the correct return type? Thank-you :)

[erictraut]

from types import TracebackType

def handle_unhandled_exception(
    exc_type: type[BaseException],
    exc_value: BaseException,
    exc_traceback: TracebackType | None,
) -> None:
    ...

[hauntsaninja]

In addition to Eric's answer, two things that could help you figure this out yourself next time:

1. Check the official typeshed types: https://github.com/python/typeshed/blob/c5dde1e7208c85cddd6cc90e8c4af733d34d1cba/stdlib/sys.pyi#L35
2. Add object types to your arguments and see what mypy has to say: def handle_unhandled_exception(exc_type: object, exc_value: object, exc_traceback: object) -> None:

...
main.py:10: error: Argument 1 to "__excepthook__" has incompatible type "object"; expected "type[BaseException]"  [arg-type]
main.py:10: error: Argument 2 to "__excepthook__" has incompatible type "object"; expected "BaseException"  [arg-type]
main.py:10: error: Argument 3 to "__excepthook__" has incompatible type "object"; expected "TracebackType | None"  [arg-type]
...

[dineshbvadhia]

Ah, I'll definitely remember this. Wish I'd known it earlier as have just spent weeks typing my code :(
Thank-you.

[Akuli]

Sometimes using int or some other obviously wrong type instead of object works better. For example, with def handle_unhandled_exception(exc_type: int, exc_value: int, exc_traceback: int) I get:

...
a.py:16: error: Incompatible types in assignment (expression has type "Callable[[int, int, int], None]", variable has type "Callable[[type[BaseException], BaseException, Optional[TracebackType]], Any]")

This error comes from the line that does sys.excepthook = handle_unhandled_exception, so the error message shows the types that the callback function needs to have. With object annotations, you actually see the types that sys.__excepthook__ expects, which happen to be the same types.

You could also use reveal_type(sys.excepthook).

[dineshbvadhia]

@Akuli Thank-you so much :)