Unable to satisfy Pyright in an except (Base)ExceptionGroup clause

[Mike Nerone (mikenerone)]

This is another problem stemming from #10217. Pyright reports the commented error for this code (command line - VSCode doesn't complain (yet?)):

exc_group: BaseExceptionGroup[BaseException]  # This line used to help, but doesn't anymore.
try:
    raise BaseExceptionGroup("A Group", [ZeroDivisionError("Hi")])
except BaseExceptionGroup as exc_group:   # -> Type of "exc_group" is "BaseExceptionGroup[Unknown]" (reportUnknownVariableType)
    print(f"BaseExceptionGroup was raised: {exc_group!r}")

The problem, of course, is the use of (Base)ExceptionGroup without specifying the generic. Simply specifying the generic satisfies Pyright, but doesn't work at runtime:

...
except BaseExceptionGroup[BaseException] as exc_group:  # -> TypeError: catching classes that do not inherit from BaseException is not allowed
,,,

I also don't see where I can typing.cast() something to work around this. I tried this, which again works at runtime, but elicits a different error from Pyright (and also adds new errors from a .subgroup() call further down in the real code):

...
except t.cast(BaseExceptionGroup[BaseException], BaseExceptionGroup) as exc_group:  # -> error: "BaseExceptionGroup[BaseException]" is not a valid exception class (reportGeneralTypeIssues)
...

Is there a solution to properly typing this, or is it the intent that all except clauses with a generic exception type need an ignore directive?

[Eric Traut (erictraut)]

Moving to discussion since this isn't a bug report but rather a question.

[Eric Traut (erictraut)]

Which version of pyright are you using? I'm not seeing the same behavior as you are. Here's the above code pasted into the pyright playground with strict-mode type checking enabled.

While the BaseExceptionGroup class is generic, it has a default type associated with the type parameter, so you shouldn't be seeing Unknown here.

[Mike Nerone (mikenerone)]

oic, then this is likely caused by the exceptiongroup backport needed for Python < 3.11 (my particular app is Python 3.8 for unavoidable reasons, but any approach that works for < 3.11 would probably apply for me, as well). This is with Pyright 1.1.399 and 1.1.400.

[Mike Nerone (mikenerone)]

Eric Traut (@erictraut) I've opened a PR for exceptiongroup that should resolve this.

[Mike Nerone (mikenerone)]

For posterity, I guess I should mention that that fix was merged and released in exceptiongroup 1.3.0 on 2025-05-10.

[luis jose guzman santos (luisjoseguzmansantos)]

yes