traceback.py: Allow customization of per-frame line formatting in StackSummary

[ammaraskar]

BPO	44569
Nosy	@terryjreedy, @ammaraskar, @pablogsal, @isidentical
PRs	bpo-44569: Decouple frame formatting in traceback.py #27038

^{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/ammaraskar'
closed_at = <Date 2021-07-16.12:21:24.792>
created_at = <Date 2021-07-05.16:47:05.034>
labels = ['library', '3.11']
title = 'traceback.py: Allow customization of per-frame line formatting in StackSummary'
updated_at = <Date 2021-07-16.12:21:24.930>
user = 'https://github.com/ammaraskar'

bugs.python.org fields:

activity = <Date 2021-07-16.12:21:24.930>
actor = 'pablogsal'
assignee = 'ammar2'
closed = True
closed_date = <Date 2021-07-16.12:21:24.792>
closer = 'pablogsal'
components = ['Library (Lib)']
creation = <Date 2021-07-05.16:47:05.034>
creator = 'ammar2'
dependencies = []
files = []
hgrepos = []
issue_num = 44569
keywords = ['patch']
message_count = 2.0
messages = ['397009', '397616']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'ammar2', 'pablogsal', 'BTaskaya']
pr_nums = ['27038']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue44569'
versions = ['Python 3.11']

[ammaraskar]

During the implementation of PEP-657, Terry Jan Reedy pointed out that in the format method of traceback.StackSummary there are two roles being fulfilled, there is some logic to handle omitting repeated lines involved in recursive calls and then the core per-frame formatting logic:

cpython/Lib/traceback.py

Lines 484 to 503 in 17f94e2

	row = []
	row.append(' File "{}", line {}, in {}\n'.format(
	frame.filename, frame.lineno, frame.name))
	if frame.line:
	row.append(' {}\n'.format(frame.line.strip()))
	stripped_characters = len(frame._original_line) - len(frame.line.lstrip())
	if frame.end_lineno == frame.lineno and frame.end_colno != 0:
	colno = _byte_offset_to_character_offset(frame._original_line, frame.colno)
	end_colno = _byte_offset_to_character_offset(frame._original_line, frame.end_colno)
	row.append(' ')
	row.append(' ' * (colno - stripped_characters))
	row.append('^' * (end_colno - colno))
	row.append('\n')
	if frame.locals:
	for name, value in sorted(frame.locals.items()):
	row.append(' {name} = {value}\n'.format(name=name, value=value))
	result.append(''.join(row))

To allow the per-line formatting to be overridden easily, these lines should be split into a separate method.

[pablogsal]

New changeset 8ce3008 by Ammar Askar in branch 'main':
bpo-44569: Decouple frame formatting in traceback.py (GH-27038)
8ce3008