Better explain the GC contract for PyType_FromSpecWithBases

[pablogsal]

BPO	44263
Nosy	@vstinner, @pablogsal, @miss-islington, @shihai1991, @erlend-aasland
PRs	bpo-44263: Better explain the GC contract for PyType_FromSpecWithBases #26442 [3.10] bpo-44263: Better explain the GC contract for PyType_FromSpecWithBases (GH-26442) #26443 [3.9] bpo-44263: Better explain the GC contract for PyType_FromSpecWithBases (GH-26442) #26444 bpo-44263: Mention PyType_Ready in the gc protocol warning #26445 [3.10] bpo-44263: Mention PyType_Ready in the gc protocol warning (GH-26445) #26446 [3.9] bpo-44263: Mention PyType_Ready in the gc protocol warning (GH-26445) #26447 bpo-44263: Py_TPFLAGS_HAVE_GC requires tp_traverse #26463 bpo-44263: Fix _decimal and _testcapi GC protocol #26464 [3.10] bpo-44263: Fix _decimal and _testcapi GC protocol (GH-26464) #26465 [3.9] bpo-44263: Fix _decimal and _testcapi GC protocol (GH-26464) #26466

^{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 2021-06-01.21:39:22.556>
created_at = <Date 2021-05-29.00:00:31.172>
labels = ['interpreter-core', 'expert-C-API', '3.11']
title = 'Better explain the GC contract for PyType_FromSpecWithBases'
updated_at = <Date 2021-06-01.21:39:22.553>
user = 'https://github.com/pablogsal'

bugs.python.org fields:

activity = <Date 2021-06-01.21:39:22.553>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2021-06-01.21:39:22.556>
closer = 'vstinner'
components = ['Interpreter Core', 'C API']
creation = <Date 2021-05-29.00:00:31.172>
creator = 'pablogsal'
dependencies = []
files = []
hgrepos = []
issue_num = 44263
keywords = ['patch']
message_count = 13.0
messages = ['394706', '394708', '394709', '394710', '394711', '394712', '394733', '394799', '394800', '394802', '394803', '394872', '394873']
nosy_count = 5.0
nosy_names = ['vstinner', 'pablogsal', 'miss-islington', 'shihai1991', 'erlendaasland']
pr_nums = ['26442', '26443', '26444', '26445', '26446', '26447', '26463', '26464', '26465', '26466']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue44263'
versions = ['Python 3.11']

[pablogsal]

We currently don't properly document how PyType_FromSpecWithBases behaves if the class is inheriting from some other class with GC support.

[pablogsal]

New changeset 8b55bc3 by Pablo Galindo in branch 'main':
bpo-44263: Better explain the GC contract for PyType_FromSpecWithBases (GH-26442)
8b55bc3

[miss-islington]

New changeset a30cbae by Miss Islington (bot) in branch '3.9':
bpo-44263: Better explain the GC contract for PyType_FromSpecWithBases (GH-26442)
a30cbae

[pablogsal]

New changeset 11049be by Miss Islington (bot) in branch '3.10':
bpo-44263: Better explain the GC contract for PyType_FromSpecWithBases (GH-26442) (GH-26443)
11049be

[pablogsal]

New changeset 43cf7c8 by Pablo Galindo in branch 'main':
bpo-44263: Mention PyType_Ready in the gc protocol warning (GH-26445)
43cf7c8

[miss-islington]

New changeset 7bd1304 by Miss Islington (bot) in branch '3.9':
bpo-44263: Mention PyType_Ready in the gc protocol warning (GH-26445)
7bd1304

[pablogsal]

New changeset d8ce746 by Miss Islington (bot) in branch '3.10':
bpo-44263: Mention PyType_Ready in the gc protocol warning (GH-26445) (bpo-26446)
d8ce746

[vstinner]

I reopen the issue. I would prefer to change PyType_FromSpecWithBases() to raise an error if a type has the Py_TPFLAGS_HAVE_GC flag but its tp_traverse function is NULL.

Oops, the Python stdlib has more types than only _ssl.SSLError with this error:

• _decimal: PyDecSignalDictMixin_Type (internal type)
• _testcapi: heapctype

[vstinner]

New changeset 142e5c5 by Victor Stinner in branch 'main':
bpo-44263: Fix _decimal and _testcapi GC protocol (GH-26464)
142e5c5

[pablogsal]

New changeset 7fe9cad by Miss Islington (bot) in branch '3.10':
bpo-44263: Fix _decimal and _testcapi GC protocol (GH-26464) (GH-26465)
7fe9cad

[miss-islington]

New changeset 46b16d0 by Miss Islington (bot) in branch '3.9':
bpo-44263: Fix _decimal and _testcapi GC protocol (GH-26464)
46b16d0

[vstinner]

New changeset ee76375 by Victor Stinner in branch 'main':
bpo-44263: Py_TPFLAGS_HAVE_GC requires tp_traverse (GH-26463)
ee76375

[vstinner]

I made sure that it's no longer possible to create a type with Py_TPFLAGS_HAVE_GC flag set but with no traverse function (tp_traverse=NULL). I close again the issue.

[vstinner]

Example of bug in the wild: Boost Python enum type defined Py_TPFLAGS_HAVE_GC flag but has no traverse function and instances doesn't have to be tracked by the GC:

• Fix enum_type_object type on Python 3.11 boostorg/python#385
• boostorg/python@a218bab

Note: it's an enum such an "enum" in C, like a Python int (1, 2, ...). It's unrelated to Python enum module.