iter(classmethod, sentinel) broken for Argument Clinic class methods?

[mjpieters]

BPO	30524
Nosy	@mjpieters, @vstinner, @serhiy-storchaka
PRs	bpo-30524: Fix _PyStack_UnpackDict() #1886 bpo-30524: Write unit tests for FASTCALL #2022 [3.6] bpo-30524: Write unit tests for FASTCALL (#2022) #2030

^{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 2017-06-09.20:45:17.849>
created_at = <Date 2017-05-31.12:01:15.958>
labels = ['interpreter-core', 'type-bug']
title = 'iter(classmethod, sentinel) broken for Argument Clinic class methods?'
updated_at = <Date 2017-06-20.20:34:33.131>
user = 'https://github.com/mjpieters'

bugs.python.org fields:

activity = <Date 2017-06-20.20:34:33.131>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2017-06-09.20:45:17.849>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2017-05-31.12:01:15.958>
creator = 'mjpieters'
dependencies = []
files = []
hgrepos = []
issue_num = 30524
keywords = ['3.6regression']
message_count = 10.0
messages = ['294835', '294836', '294841', '294845', '294847', '295515', '295535', '295566', '295572', '296486']
nosy_count = 3.0
nosy_names = ['mjpieters', 'vstinner', 'serhiy.storchaka']
pr_nums = ['1886', '2022', '2030']
priority = 'high'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30524'
versions = ['Python 3.6']

[mjpieters]

I'm not sure where exactly the error lies, but bpo-27128 broke iter() for Argument Clinic class methods. The following works in Python 3.5, but not in Python 3.6:

from datetime import datetime
from asyncio import Task

next(iter(datetime.now, None))
next(iter(Task.all_tasks, None))

In 3.6 StopIteration is raised:

>>> next(iter(datetime.now, None))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>> next(iter(Task.all_tasks, None))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

(In 3.5 a datetime.datetime and set object are produced, respectively)

The only thing these two methods have in common is that they are class methods with no arguments, parsed out by the Argument Clinic generated code (so using _PyArg_Parser).

What appears to have changed is that iter() was switched from using PyObject_Call to _PyObject_FastCall, see 99ee9c7

[mjpieters]

Forgot to addthis: this bug was found via https://stackoverflow.com/questions/44283540/iter-not-working-with-datetime-now

[serhiy-storchaka]

It works in 3.5 and 3.7, but raises a StopIteration in 3.6.

[vstinner]

Oh, it's my fault: it's a bug coming from FASTCALL optimizations. The strange thing is that the bug wasn't catched by the giant Python test suite :-(

I knew that _PyStack_UnpackDict() has a bug in Python 3.6, but I completely forgot to fix it :-(

#1886 fixes the bug, but has not test yet.

[serhiy-storchaka]

next(iter(datetime.now, None)) can be turned into a nice test.

[vstinner]

New changeset f0ff849 by Victor Stinner in branch '3.6':
bpo-30524: Fix _PyStack_UnpackDict() (bpo-1886)
f0ff849

[vstinner]

New changeset 3b5cf85 by Victor Stinner in branch 'master':
bpo-30524: Write unit tests for FASTCALL (bpo-2022)
3b5cf85

[vstinner]

New changeset b757745 by Victor Stinner in branch '3.6':
bpo-30524: Write unit tests for FASTCALL (bpo-2022) (bpo-2030)
b757745

[vstinner]

Sorry for the regression, sadly, it wasn't catch before by any test. I added a lot of new tests, so we should cover more cases. Oh, and the bug has been fixed in 3.6 :-)

[vstinner]

bpo-30473 has been marked as a duplicate of this bug.