Mailman 3 PEP 654: Exception Groups and except* [REPOST] - Python-Dev

20 Mar 2021

      We would like to present for feedback a new version of PEP 654, which
incorporates the feedback we received in the discussions so far:
https://www.python.org/dev/peps/pep-0654/
The reference implementation has also been updated along with the PEP.

The changes we made since the first post are:

1. Instead of ExceptionGroup(BaseException), we will have two new builtin
types: BaseExceptionGroup(BaseException) and
ExceptionGroup(BaseExceptionGroup, Exception).
This is so that "except Exception" catches ExceptionGroups (but not
BaseExceptionGroups). BaseExceptionGroup.__new__ inspects the wrapped
exceptions, and if they are all Exception subclasses, it creates an
ExceptionGroup instead of a BaseExceptionGroup.

2. The exception group classes are not final - they can be subclassed and
split()/subgroup() work correctly if the subclass overrides the derive()
instance method as described here:
https://www.python.org/dev/peps/pep-0654/#subclassing-exception-groups

3. We had some good suggestions on formatting exception groups, which we
have implemented as you can see in the output shown for the examples in the
PEP.

4. We expanded the section on handling Exception Groups, to show how
subgroup can be used (with side effects) to do something for each leaf
exception, and how to iterate correctly when the tracebacks of leaf
exceptions are needed:
https://www.python.org/dev/peps/pep-0654/#handling-exception-groups

5. We expanded the sections on rationale and backwards compatibility to
explain our premise and expectations regarding how exception groups will be
used and how the transition to using them will be managed.

6. We added several items to the rejected ideas section.

We did not receive any comments (or make any changes) to the proposed
semantics of except*, hopefully this is because everyone thought they are
sensible.

Irit, Yury and Guido

PEP 654: Exception Groups and except* [REPOST]

Irit Katriel

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