TracebackException or StackSummary.extract with capture_locals=True fail to catch exceptions raised by repr() on value of frame local variable in FrameSummary.__init__.

[moi90]

BPO	43656
Nosy	@rbtcollins, @serhiy-storchaka, @iritkatriel, @moi90, @akulakov
PRs	bpo-43656: Make FrameSummary robust towards un-repr-able values #25062 bpo-43656: Introduce format_locals in traceback #29299

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

Show more details

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-03-29.09:36:49.757>
labels = ['invalid', 'type-feature', '3.8', '3.9', '3.10']
title = 'TracebackException or StackSummary.extract with capture_locals=True fail to catch exceptions raised by repr() on value of frame local variable in FrameSummary.__init__.'
updated_at = <Date 2021-11-25.14:52:05.471>
user = 'https://github.com/moi90'

bugs.python.org fields:

activity = <Date 2021-11-25.14:52:05.471>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-03-29.09:36:49.757>
creator = 'moi90'
dependencies = []
files = []
hgrepos = []
issue_num = 43656
keywords = ['patch']
message_count = 43.0
messages = ['389679', '389680', '389794', '389815', '389819', '389840', '389842', '393199', '393298', '393519', '404319', '404330', '404392', '404445', '404452', '404456', '404457', '404464', '404468', '404514', '404567', '404603', '404736', '404950', '404988', '404989', '405028', '405033', '405049', '405214', '405238', '405893', '405918', '405935', '405994', '406008', '406012', '406042', '406107', '406997', '406998', '406999', '407001']
nosy_count = 6.0
nosy_names = ['rbcollins', 'serhiy.storchaka', 'iritkatriel', 'moi90', 'andrei.avk', 'jbw']
pr_nums = ['25062', '29299']
priority = 'normal'
resolution = 'not a bug'
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43656'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[moi90]

With capture_locals=True, StackSummary.format prints the local variables for every frame:

cpython/Lib/traceback.py

Line 440 in 4827483

if frame.locals:

This will fail, however, if string conversion fails.

StackSummary.format should be robust towards such possibilities.

An easy fix would be a utility function:

def try_str(x):
  try:
    return str(x)
  except:
    return "<some sensible hint>"

[moi90]

I have to correct myself: The conversion to string already happens during construction, in FrameSummary.__init__:

cpython/Lib/traceback.py

Line 273 in 4827483

self.locals = {k: repr(v) for k, v in locals.items()} if locals else None

The issue remains as severe and the fix remains as easy.

[serhiy-storchaka]

Why str() is called at all? Would not be better to use repr()?

[moi90]

Yes, it is repr in FrameSummary.__init__.

[iritkatriel]

See bpo-39228.