traceback.extract_tb breaks compatibility by returning FrameSummary

[encukou]

BPO	26502
Nosy	@rbtcollins, @bitdancer, @encukou, @berkerpeksag
PRs	bpo-26502: Implement FrameSummary.__len__() #8632

^{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 2018-09-11.17:54:10.821>
created_at = <Date 2016-03-07.11:15:48.241>
labels = ['3.8', 'type-bug', 'library']
title = 'traceback.extract_tb breaks compatibility by returning FrameSummary'
updated_at = <Date 2018-09-11.17:54:10.819>
user = 'https://github.com/encukou'

bugs.python.org fields:

activity = <Date 2018-09-11.17:54:10.819>
actor = 'berker.peksag'
assignee = 'docs@python'
closed = True
closed_date = <Date 2018-09-11.17:54:10.821>
closer = 'berker.peksag'
components = ['Library (Lib)']
creation = <Date 2016-03-07.11:15:48.241>
creator = 'petr.viktorin'
dependencies = []
files = []
hgrepos = []
issue_num = 26502
keywords = ['patch']
message_count = 7.0
messages = ['261296', '261357', '275356', '324937', '325036', '325038', '325040']
nosy_count = 5.0
nosy_names = ['rbcollins', 'r.david.murray', 'petr.viktorin', 'docs@python', 'berker.peksag']
pr_nums = ['8632']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue26502'
versions = ['Python 3.8']

[encukou]

According to the docs [0], traceback.extract_tb should return 4-tuples (filename, line number, function name, text).

[0] https://docs.python.org/3/library/traceback.html#traceback.extract_tb

Instead, since Python 3.5, it returns FrameSummary objects, which are not tuples, nor tuple subclasses, nor even sequences. (This broke some code in the wild that called len() on FrameSummary.)

bpo-25111 pointed out another tuple incompatibility, which was fixed. Should __len__ be added as well? Or the whole Sequence ABC?

I can provide a patch when those questions are answered.

[berkerpeksag]

This is basically a duplicate of bpo-25573. We need to add the examples in traceback module documentation to test_traceback.py.

Should __len__ be added as well?

IMO, the new FrameSummary API (and its friends) shouldn't change the return value of the existing traceback functions like extract_tb() so I'd say yes.

[bitdancer]

I disagree that this is a duplicate. The question here is, should we add __len__? Since a FrameSummary object can be "cast into a tuple" and has a known len, I would say the answer is definitely yes.

[encukou]

New changeset 9797b7a by Petr Viktorin (Berker Peksag) in branch 'master':
bpo-26502: Implement FrameSummary.__len__() (GH-8632)
9797b7a

[berkerpeksag]

Thanks for merging this, Petr. What do you think about backporting to 3.7?

[encukou]

I don't think I have a good enough feel for what should, generally, go to 3.7. So take my opinion with a grain of salt.

But honestly, I don't think the issue is important enough -- I've only seen one codebase affected by it, which wasn't hard to fix.

[berkerpeksag]

Ok, let's close this issue then.