Better str() for BaseExceptionGroup

[iritkatriel]

BPO	46729
Nosy	@gvanrossum, @1st1, @iritkatriel
PRs	bpo-46729: improved str() for BaseExceptionGroup #31294

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2022-02-22.18:30:58.471>
created_at = <Date 2022-02-12.11:02:01.480>
labels = ['interpreter-core', '3.11']
title = 'Better str() for BaseExceptionGroup'
updated_at = <Date 2022-02-22.18:30:58.469>
user = 'https://github.com/iritkatriel'

bugs.python.org fields:

activity = <Date 2022-02-22.18:30:58.469>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2022-02-22.18:30:58.471>
closer = 'iritkatriel'
components = ['Interpreter Core']
creation = <Date 2022-02-12.11:02:01.480>
creator = 'iritkatriel'
dependencies = []
files = []
hgrepos = []
issue_num = 46729
keywords = ['patch', '3.2regression']
message_count = 3.0
messages = ['413121', '413138', '413735']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'yselivanov', 'iritkatriel']
pr_nums = ['31294']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46729'
versions = ['Python 3.11']

[iritkatriel]

The str() of exception groups currently contains just the msg as passed to the constructor. This turned out to be confusing (see #31270 (comment)).

We should consider whether it is possible to design a more informative str().

Note that the str() is included in the standard traceback, which include the line:

f"{type(e)}: {str(e)}"

So str() should not repeat the type, and should not clutter this too much. Probably just the msg plus the number of contained leaf exceptions.

PEP-654 needs to be updated with what we do here, and the change needs to be approved by the SC.

[gvanrossum]

Should it show the number of immediate subexceptions (i.e., len(eg.exceptions)) or the number of leaf exceptions? I'd be happy with the former (since that's what EdgeDb's MultiError does), which would not require adding an extra field to the base exception group struct.

[iritkatriel]

New changeset 38b5acf by Irit Katriel in branch 'main':
bpo-46729: add number of sub-exceptions in str() of BaseExceptionGroup (GH-31294)
38b5acf