IDLE: Fix reporting RE error in searchengine

[serhiy-storchaka]

BPO	42426
Nosy	@terryjreedy, @serhiy-storchaka, @miss-islington
PRs	bpo-42426: IDLE: Fix reporting offset of the RE error in searchengine #23447 [3.9] bpo-42426: IDLE: Fix reporting offset of the RE error in searchengine (GH-23447) #23453 [3.8] bpo-42426: IDLE: Fix reporting offset of the RE error in searchengine (GH-23447) #23454

^{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 = 'https://github.com/terryjreedy'
closed_at = <Date 2020-11-22.05:56:33.933>
created_at = <Date 2020-11-21.15:19:37.261>
labels = ['expert-IDLE', 'type-bug', '3.8', '3.9', '3.10']
title = 'IDLE: Fix reporting RE error in searchengine'
updated_at = <Date 2020-11-22.05:56:33.932>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2020-11-22.05:56:33.932>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = True
closed_date = <Date 2020-11-22.05:56:33.933>
closer = 'terry.reedy'
components = ['IDLE']
creation = <Date 2020-11-21.15:19:37.261>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 42426
keywords = ['patch']
message_count = 5.0
messages = ['381566', '381586', '381588', '381591', '381594']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'serhiy.storchaka', 'miss-islington']
pr_nums = ['23447', '23453', '23454']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42426'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[serhiy-storchaka]

In Lib/idlelib/searchengine.py there is a code which decomposes re.error and reports error message, pattern, and position of error.

The problem is that that code does not match the structure of re.error. It takes args[0] as error message, and args[1] (if specified) as position in the pattern. But actually re.error always sets args[0] to formatted error message, and args has only one item. So some code in IDLE is dead.

Currently reported:

Error: bad escape \z at position 1
Pattern: a\z

Expected:

Error: bad escape \z
Pattern: a\z
Offset: 1

[terryjreedy]

The confusion about error.args is this:

BaseException.args is documented as "The tuple of arguments given to the exception constructor."
https://docs.python.org/3/library/exceptions.html#base-classes
args[0] is usually and I would expect it to always be the formatted message, which is to say, the string normally printed at the end of tracebacks.
The signature of re.error is documented as "(msg, pattern=None, pos=None)"
https://docs.python.org/3/library/re.html#re.error
and this is what inspect.signature says (as reported in IDLE calltip).
So one might expect .args to be a 3-tuple.

With the value fetching fixed, the displayed message could be changed to something else (in another issue), such as
Error: bad escape \z in pattern r'a\z' at offset 1

Thanks for the patch.

[terryjreedy]

New changeset 453bc1d by Serhiy Storchaka in branch 'master':
bpo-42426: IDLE: Fix reporting offset of the RE error in searchengine (GH-23447)
453bc1d

[miss-islington]

New changeset 28b40d7 by Miss Islington (bot) in branch '3.8':
bpo-42426: IDLE: Fix reporting offset of the RE error in searchengine (GH-23447)
28b40d7

[miss-islington]

New changeset dd20643 by Miss Islington (bot) in branch '3.9':
bpo-42426: IDLE: Fix reporting offset of the RE error in searchengine (GH-23447)
dd20643