Clarifying typing behavior for exception handlers (except/except*)

[Jelle (Jelle Zijlstra)]

Welcome, and thanks for the detailed post!

@yangdanny97

If this is intentional to avoid confusing beginners then never mind, but if it’s a mistake then I think the “parenthesized” should be removed.

I think the “parenthesized” part here is because except A, B: is a SyntaxError, to avoid confusion with the old Python 2 syntax (where you wrote except Exception, e: instead of except Exception as e:).

Possibly this wording can be improved, but I wouldn’t look to the tutorial for a fully precise specification of the behavior in rarely seen cases. You also link the wording in the language reference, and there the wording appears to be mostly correct.

@yangdanny97

An except* clause must have a matching type, and this type cannot be a subclass of BaseExceptionGroup.

I agree this is imprecise and should mention that tuples are allowed. Could you open an issue and make a PR to the language reference?

@yangdanny97

The typing spec doesn’t seem to have anything about exception handler typechecking in the exceptions section.

I don’t want the typing spec to duplicate the language reference; in cases like this, the implicit spec is that type checkers should follow the behavior specified in the language reference.

@yangdanny97

2. Should typecheckers check against types that extend BaseExceptionGroup in except*?

I think ideally type checkers should give an error if the except* clause contains a literal class or tuple of literal classes and one of them is a subclass of ExceptionGroup, yes.

@yangdanny97

3. If yes for #2, should the type system disallow values which could be BaseExceptionGroup (like a type variable of type[BaseException]) in except*?

Note your example uses a type alias, not a type variable.

I think disallowing your example would be more trouble than it’s worth. The type system has no way to express difference types.