PEP 678: Make it possible to enrich an exception's error message

[iritkatriel]

BPO	45607
Nosy	@gvanrossum, @aroberge, @Zac-HD, @iritkatriel
PRs	bpo-45607: Make it possible to enrich exception displays via setting their __note__ field #29880

^{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 2021-12-03.22:02:13.816>
created_at = <Date 2021-10-25.20:38:01.298>
labels = ['interpreter-core', 'type-feature', '3.11']
title = "Make it possible to enrich an exception's error message"
updated_at = <Date 2021-12-03.22:02:13.816>
user = 'https://github.com/iritkatriel'

bugs.python.org fields:

activity = <Date 2021-12-03.22:02:13.816>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2021-12-03.22:02:13.816>
closer = 'iritkatriel'
components = ['Interpreter Core']
creation = <Date 2021-10-25.20:38:01.298>
creator = 'iritkatriel'
dependencies = []
files = []
hgrepos = []
issue_num = 45607
keywords = ['patch']
message_count = 4.0
messages = ['404999', '405016', '407365', '407607']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'aroberge', 'Zac Hatfield-Dodds', 'iritkatriel']
pr_nums = ['29880']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue45607'
versions = ['Python 3.11']

[iritkatriel]

The requirement comes from Hypothesis, see
#28569 (comment)

It is necessary there to add a note to an exception describing which test case it comes from. The note should be printed by __str__ of this exception.

class Explanation(Exception):
 __module__ = "builtins"
 def __str__(self) -> str:
 return f"\n{self.args[0]}"

try:
why = "Failed!"
raise AssertionError(why)
except Exception as e:
msg = " You can reproduce this error by ...\n ..."
raise Explanation(msg) from e

 # Ideally something more like:
 e.__note__ = msg
 raise

[Zac-HD]

This code shows my current best workaround based on a wrapper exception, with the traceback below annotating the additional details that I'd prefer to omit for clarity:

$ python example.py
Traceback (most recent call last):
 File "example.py", line 8, in <module>
 raise AssertionError(why)
AssertionError: Failed!
 # These lines are
The above exception was the direct cause of the following exception: # confusing for new 
 # users, and they
Traceback (most recent call last): # only exist due 
 File "example.py", line 10, in <module> # to implementation
 raise Explanation(msg) from e # via the Explanation
Explanation:  # wrapper type :-(
 You can reproduce this error by ...
 ...

The motivation for this is that we'd like to use ExceptionGroup to indicate that MultipleFailures is a group of exceptions, and replace our current print()-based method of reporting the details of the inner exceptions.

[iritkatriel]

bpo-28953 is another use case for this feature.

[iritkatriel]

New changeset 5bb7ef2 by Irit Katriel in branch 'main':
bpo-45607: Make it possible to enrich exception displays via setting their __note__ field (GH-29880)
5bb7ef2

[iritkatriel]

Reopening to implement PEP 678.