test.test_getargs2.TupleSubclass test failure

[rbtcollins]

BPO	28086
Nosy	@warsaw, @rhettinger, @gpshead, @rbtcollins, @ned-deily, @serhiy-storchaka, @zooba, @s-t-e-v-e-n-k
PRs	[Do Not Merge] Convert Misc/NEWS so that it is managed by towncrier #552
Files	issue28086.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/serhiy-storchaka'
closed_at = <Date 2016-09-22.16:45:30.712>
created_at = <Date 2016-09-11.22:43:57.027>
labels = ['interpreter-core', 'deferred-blocker', 'type-bug', '3.7']
title = 'test.test_getargs2.TupleSubclass test failure'
updated_at = <Date 2017-03-31.16:36:22.323>
user = 'https://github.com/rbtcollins'

bugs.python.org fields:

activity = <Date 2017-03-31.16:36:22.323>
actor = 'dstufft'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2016-09-22.16:45:30.712>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2016-09-11.22:43:57.027>
creator = 'rbcollins'
dependencies = []
files = ['44785']
hgrepos = []
issue_num = 28086
keywords = ['patch']
message_count = 10.0
messages = ['275879', '275882', '275936', '275956', '275961', '276066', '276071', '277213', '277227', '277232']
nosy_count = 9.0
nosy_names = ['barry', 'rhettinger', 'gregory.p.smith', 'rbcollins', 'ned.deily', 'python-dev', 'serhiy.storchaka', 'steve.dower', 'stevenk']
pr_nums = ['552']
priority = 'deferred blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28086'
versions = ['Python 3.6', 'Python 3.7']

[rbtcollins]

The test.test_getargs2.TupleSubclass test is failing in master. I suspect its a fastcall fallout.

[s-t-e-v-e-n-k]

I've bisected this via the git mirror, and converting it to the hg changeset gives changeset 103659:51b635e81958 as what introduced this failure.

[zooba]

I added a fix in 7793d34609cb assuming that the intent is to allow subclasses of tuple to be passed directly as *args rather than creating a new tuple. The original changeset seems to suggest this is the intent.

[ned-deily]

Serhiy, what do you think?

[serhiy-storchaka]

Thank you for making buildbots green Steve.

The intent of 51b635e81958 was avoiding needless copying. But this caused to leaking tuple subtype in case of single var-positional argument without other positional arguments. The intent of tests was checking that tuple subtype is not leaked to called function. There is a bug, but I need to find the place where tuple subtype should be converted to tuple without needless hitting performance.

[gpshead]

I'm marking this as a deferred blocker as i believe we want this resolved before we exit the betas.

[serhiy-storchaka]

This bug is not so critical. This bug exists in 3.5 and 2.7. Tests was added in bpo-18531, but at that moment the bug already was fixed in 3.6 (maybe accidentally).

[serhiy-storchaka]

The simplest way is just change PyTuple_Check to PyTuple_CheckExact in ceval.c. Maybe this is suboptimal for tuple subclasses (namedtuple?), but I think this is very rare case.