Passing keyword arguments to types.GenericAlias causes a hard crash

[Fidget-Spinner]

BPO	42576
Nosy	@gvanrossum, @miss-islington, @Fidget-Spinner
PRs	bpo-42576: Raise TypeError when passing in keyword arguments to GenericAlias #23656 [3.9] bpo-42576: Raise TypeError when passing in keyword arguments to GenericAlias (GH-23656) #23659 bpo-42576: Clarify only debug builds are affected in news #23663 [3.9] bpo-42576: Clarify only debug builds are affected in news (GH-23663) #23666

^{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 2020-12-05.16:28:26.882>
created_at = <Date 2020-12-05.14:31:19.254>
labels = ['interpreter-core', '3.9', '3.10']
title = 'Passing keyword arguments to types.GenericAlias causes a hard crash'
updated_at = <Date 2020-12-06.17:01:05.979>
user = 'https://github.com/Fidget-Spinner'

bugs.python.org fields:

activity = <Date 2020-12-06.17:01:05.979>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2020-12-05.16:28:26.882>
closer = 'kj'
components = ['Interpreter Core']
creation = <Date 2020-12-05.14:31:19.254>
creator = 'kj'
dependencies = []
files = []
hgrepos = []
issue_num = 42576
keywords = ['patch']
message_count = 8.0
messages = ['382565', '382571', '382573', '382574', '382578', '382591', '382600', '382602']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'miss-islington', 'kj']
pr_nums = ['23656', '23659', '23663', '23666']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue42576'
versions = ['Python 3.9', 'Python 3.10']

[Fidget-Spinner]

I noticed that passing keyword arguments to types.GenericAlias's __new__ causes the interpreter to hard crash and exit due to an assertion failure:

import types
types.GenericAlias(bad=float)

Result:
Assertion failed: PyTuple_CheckExact(kwnames), file ....\cpython\Python\getargs.c, line 2767

A simple fix is to just use _PyArg_NoKeywords instead of _PyArg_NoKwnames.

Looking through the rest of the C code, it seems that apart from GenericAlias, only vectorcalls for various builtins use _PyArg_NoKwnames. However, they don't seem to be affected by the bug.

[gvanrossum]

Oh, this looks like I copied the wrong idiom. Good catch!

[miss-islington]

New changeset 804d689 by kj in branch 'master':
bpo-42576: Raise TypeError when passing in keyword arguments to GenericAlias (GH-23656)
804d689

[miss-islington]

New changeset d5c029b by Miss Islington (bot) in branch '3.9':
bpo-42576: Raise TypeError when passing in keyword arguments to GenericAlias (GH-23656)
d5c029b

[gvanrossum]

Thanks for the quick fix!

[Fidget-Spinner]

Thanks for taking the time to review this! I forgot to specify this only affects debug builds in the news entry. I'm opening a PR to edit that because I don't want people to mistake it for an actual interpreter-crashing bug in the release build (that would probably cause some amount of unneeded worry).

[miss-islington]

New changeset 6a7fb9d by kj in branch 'master':
bpo-42576: Clarify only debug builds are affected in news (GH-23663)
6a7fb9d

[miss-islington]

New changeset 8502d46 by Miss Islington (bot) in branch '3.9':
[3.9] bpo-42576: Clarify only debug builds are affected in news (GH-23663) (GH-23666)
8502d46