Make sure exceptions raised in __aiter__ are properly chained in ceval

[1st1]

BPO	28893
Nosy	@brettcannon, @vstinner, @ned-deily, @serhiy-storchaka, @1st1
PRs	bpo-28893: Set __cause__ for errors in async iteration protocol #407 bpo-28893: Set __cause__ for errors in async iteration protocol #414 [Do Not Merge] Sample of CPython life with blurb. #703 [Do Not Merge] Convert Misc/NEWS so that it is managed by towncrier #552
Files	chain.patch

^{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/1st1'
closed_at = <Date 2017-03-03.23:10:38.759>
created_at = <Date 2016-12-07.04:41:33.466>
labels = ['interpreter-core', 'type-bug', '3.7']
title = 'Make sure exceptions raised in __aiter__ are properly chained in ceval'
updated_at = <Date 2017-03-31.16:36:30.278>
user = 'https://github.com/1st1'

bugs.python.org fields:

activity = <Date 2017-03-31.16:36:30.278>
actor = 'dstufft'
assignee = 'yselivanov'
closed = True
closed_date = <Date 2017-03-03.23:10:38.759>
closer = 'yselivanov'
components = ['Interpreter Core']
creation = <Date 2016-12-07.04:41:33.466>
creator = 'yselivanov'
dependencies = []
files = ['45782']
hgrepos = []
issue_num = 28893
keywords = ['patch']
message_count = 6.0
messages = ['282595', '282597', '282659', '282675', '288853', '290339']
nosy_count = 5.0
nosy_names = ['brett.cannon', 'vstinner', 'ned.deily', 'serhiy.storchaka', 'yselivanov']
pr_nums = ['407', '414', '703', '552']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28893'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7']

[1st1]

Originally reported in http://bugs.python.org/issue28885#msg282538

Ned, please take a look at the attached patch. While I think that the change is quite small & safe to merge, I think it's OK if we push this only in 3.6.1. It's a nice usability improvement, but only for people that implement objects with custom __await__, which is quite a rare case.

[serhiy-storchaka]

Should the original exception be stored on the __context__ or the __cause__ attribute? In the latter case you should use _PyErr_FormatFromCause().

[ned-deily]

Yury, sounds like a bug to me so it would be appropriate for a maintenance release. And not a showstopper release critical.

[1st1]

Ned, sounds good.

Serhiy, we don't have _PyErr_FormatFromCause in 3.5, so I'll probably commit the patch as is in 3.5. For 3.6 is definitely makes sense to use _PyErr_FormatFromCause.

[1st1]

New changeset dea5101 by Yury Selivanov in branch '3.6':
bpo-28893: Set __cause__ for errors in async iteration protocol (#407)
dea5101

[1st1]

New changeset 398ff91 by Yury Selivanov in branch 'master':
bpo-28893: Set __cause__ for errors in async iteration protocol (#407)
398ff91