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PyList_SET_ITEM could be safer #74644
Description
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 2020-12-07.10:57:40.893>
created_at = <Date 2017-05-24.15:14:12.544>
labels = ['interpreter-core', 'type-feature', '3.10']
title = 'PyList_SET_ITEM could be safer'
updated_at = <Date 2021-10-15.17:44:47.694>
user = 'https://bugs.python.org/espie'bugs.python.org fields:
activity = <Date 2021-10-15.17:44:47.694>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2020-12-07.10:57:40.893>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2017-05-24.15:14:12.544>
creator = 'espie'
dependencies = []
files = []
hgrepos = []
issue_num = 30459
keywords = ['patch']
message_count = 23.0
messages = ['294362', '294421', '294450', '294451', '294472', '294490', '294499', '294502', '294507', '294511', '382550', '382560', '382561', '382562', '382626', '382627', '382825', '382830', '382831', '382832', '382971', '384058', '404040']
nosy_count = 8.0
nosy_names = ['rhettinger', 'vstinner', 'petr.viktorin', 'skrah', 'serhiy.storchaka', 'Christian.Tismer', 'espie', 'ZackerySpytz']
pr_nums = ['19975', '23645', '23654', '28982']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30459'
versions = ['Python 3.10']
Activity
espie commentedon May 24, 2017
Documentation says PyList_SET_ITEM is void, but it lies. The macro is such that it yields the actual element being set.
I just ran into the problem while compiling py-qt4 with clang.
There was some confusion between PyList_SET_ITEM and PyList_SetItem:
if (obj == NULL || PyList_SET_ITEM (l, i, obj) < 0)
g++ didn't catch it (because it doesn't see negative pointers as a problem), but clang++ instantly broke.
With PyList_SET_ITEM truly void the problem disappears.
rhettinger commentedon May 25, 2017
The docs do make the claim of returning void: https://docs.python.org/3/c-api/list.html#c.PyList_SET_ITEM
However, I think we should change the docs rather than changing the macro. This macro is very old and very widely used. Changing it is likely to break existing code which may rely on the current behavior.
Also, I don't want to encourage code like what you saw in py-qt4. It is very indirect about what it is trying to express.
espie commentedon May 25, 2017
Well, there is not going to be a lot of breakage. This problem is the only instance I've encountered in the full OpenBSD ports tree.
I thought python was supposed to be a clean language, and didn't shy away from removing stuff/tweaking stuff to achieve that goal ?...
The py-kde4 error was deadly. I'm lucky clang finally caught it, but I'd rather this kind of stuff just not compile.
I think we're in a world where *correctness* is finally beginning to matter a bit more than *compatibility forever whatever the cost*.
serhiy-storchaka commentedon May 25, 2017
I think it would be safer to just cast the result of the expression to void. This decreases the chance of breaking valid code, that for example uses PyList_SET_ITEM() in a comma expression.
espie commentedon May 25, 2017
yep, casting to (void) would be safer indeed. didn't think of that one ;)
rhettinger commentedon May 25, 2017
That is no basis for knowing what the entire rest of the world has done with done with this API (perhaps valid working code using chained assignments).
espie commentedon May 25, 2017
Note that the API is fully documented for returning void... not anything else.
"No basis" right. We're taling 10000 pieces of software. a lot of what is actually used in the world.
I'm very surprised, considering python has routinely done "spring cleanup" by breaking fairly old apis.
If this breaks, people will fix their code, seriously.
In most places, we would rather have undocumented, unportable code, break *cleanly*, rather than rely on a fuzzy behavior that could possibly change at any moment, and that was never documented as doing anything.
Or is there some kind of mystique that, because this is low-level C implementation, somehow, python programmers are not going to be able to cope with the internals ?....
skrah commentedon May 25, 2017
I guess since it's documented, it would be ok to change for 3.7. BTW, we also allow "static inline" now in headers, so perhaps we could make it a function.
rhettinger commentedon May 25, 2017
I prefer to leave this as macro rather than assuming the compiler will heed the inline hint.
espie commentedon May 25, 2017
it's still 100% safe as a macro since each parameter is not used more than once. only the return type is an issue.
2 remaining items
serhiy-storchaka commentedon Dec 5, 2020
I prefer PR 19975 to PR 23645. It solves the initial issue and is much simpler.
vstinner commentedon Dec 5, 2020
New changeset 556d97f by Zackery Spytz in branch 'master':
bpo-30459: Cast the result of PyList_SET_ITEM() to void (GH-19975)
556d97f
vstinner commentedon Dec 7, 2020
New changeset 0ef96c2 by Victor Stinner in branch 'master':
bpo-30459: Cast the result of PyCell_SET to void (GH-23654)
0ef96c2
vstinner commentedon Dec 7, 2020
Thanks Espie Marc for the bug report, it's now fixed in the master branch. IMO not only clang users will benefit of a better defined API. For example, it should help other Python implementation to implement such API, without the weird side effects of a macro.
encukou commentedon Dec 10, 2020
This change goes directly against PEP-387.
vstinner commentedon Dec 10, 2020
The change respects the documentation which always documented the result type as "void".
3.10: https://docs.python.org/dev/c-api/tuple.html#c.PyTuple_SET_ITEM
3.5: https://docs.python.org/3.5/c-api/tuple.html#c.PyTuple_SET_ITEM
2.7: https://docs.python.org/2.7/c-api/tuple.html#c.PyTuple_SET_ITEM
This change is backward incompatible on purpose: it's to implement the documented behavior, and the macro was misused leading to a bug: "PyList_SET_ITEM (l, i, obj) < 0" in py-qt4.
I would also prefer a deprecation warning, but I don't see how do detect when a macro is abused to write "x = SET();" or "SET() = x;" Maybe a C linter could detect that, but I don't know any tool doing that.
The best we can do is to announce the change. That's why I documented in What's New in Python 3.10, and not only "hidden" in the Changelog:
https://docs.python.org/dev/whatsnew/3.10.html#id2
My expectation is that apart py-qt4, no project abuse these 3 macros.
vstinner commentedon Dec 10, 2020
I checked: commit 0ef96c2 is part of Python 3.10.0a3 (released 3 days ago).
encukou commentedon Dec 10, 2020
Then, IMO, the documentation should be fixed.
That's not true; at least ALSA's python bindings use PyTuple_SET_ITEM as a lvalue as well.
vstinner commentedon Dec 14, 2020
alsa-python used PyTuple_SET_ITEM(..., obj) to decide if it should call Py_INCREF(obj). This code looks suspicious. PyTuple_SET_ITEM() should not be used to set an item to NULL.
It's already fixed:
alsa-project/alsa-python@5ea2f87
IMO it's a good thing that such suspicious code is discovered. The surprising part is that it worked previously :-)
Downstream Fedora issue: https://bugzilla.redhat.com/show_bug.cgi?id=1906380 (CLOSED)
ctismer commentedon Dec 30, 2020
Congrats to that change!
vstinner commentedon Oct 15, 2021
New changeset 51f8196 by Victor Stinner in branch 'main':
bpo-30459: Use (void) in macros setting variables (GH-28982)
51f8196