[Typing-sig] Re: type hint of function exceptions

March 12, 2023


      The first version can be simplified with a tuple of exceptions rather than a union:

```
from typing import NoReturn


class SomeError(Exception):
    ...


class AnotherError(Exception):
    ...


class StillAnotherError(Exception):
    ...


def assert_never(value: NoReturn) -> NoReturn:
    # This also works at runtime as well
    assert False, f"This code should never be reached, got: {value}"


FooError = (ValueError, SomeError, AnotherError)


def foo(i: int):
    if i == 0:
        raise ValueError("Must not be zero")
    elif i == 1:
        raise SomeError("Must not be one")
    elif i == 2:
        raise AnotherError("Must not be two")
    print("that's ok")


try:
    foo(0)
except FooError as e:
    if isinstance(e, ValueError):
        print("value error")
    elif isinstance(e, SomeError):
        print("some error")
    else:
        # mypy successfully check for exhaustiveness:  Argument 1 to "assert_never" has incompatible type "AnotherError"; expected "NoReturn"  [arg-type]
        assert_never(e)
        raise
```

[Typing-sig] Re: type hint of function exceptions

david.froger＠mailoo.org