gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases by encukou · Pull Request #151252 · python/cpython · GitHub

The initial implementation of PEP 820 worsened the error message when non-types are given as base types in Py_tp_bases & Py_tp_base (as Peter noted in the review).
Bring back the 'bases must be types' wording and add a 'got' note for easier debugging.

Improve slot ID documentation, and soft-deprecate Py_tp_base (as per the PEP).
Add more tests.

Issue: Implement PEP 820 – PySlot: Unified slot system for the C API #149044

…ype bases The initial implementation of PEP 820 worsened the error message when non-types are given as base types in Py_tp_bases & Py_tp_base. Bring back the 'bases must be types' wording and add a 'got' note for easier debugging. Improve slot ID documentation, and soft-deprecate Py_tp_base (as per the PEP).

Documentation build overview

📚 cpython-previews | 🛠️ Build #33091696 | 📁 Comparing 1de19c5 against main (8c0e251)

🔍 Preview build

25 files changed · + 1 added · ± 24 modified

+ Added

library/asyncio-threading.html

± Modified

Thanks @encukou for the PR 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

GH-151606 is a backport of this pull request to the 3.15 branch.

…ype bases (pythonGH-151252) The initial implementation of PEP 820 worsened the error message when non-types are given as base types in Py_tp_bases & Py_tp_base. Bring back the 'bases must be types' wording and add a 'got' note for easier debugging. Improve slot ID documentation, and soft-deprecate Py_tp_base (as per the PEP).

…type bases (GH-151252) (GH-151606) gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases (GH-151252) The initial implementation of PEP 820 worsened the error message when non-types are given as base types in Py_tp_bases & Py_tp_base. Bring back the 'bases must be types' wording and add a 'got' note for easier debugging. Improve slot ID documentation, and soft-deprecate Py_tp_base (as per the PEP). (cherry picked from commit 16185e9) Co-authored-by: Petr Viktorin <encukou@gmail.com>

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Debian root 3.15 (tier-1) has failed when building commit e9c7cc1.

What do you need to do:

Don't panic.
Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/2019/builds/205) and take a look at the build logs.
Check if the failure is related to this commit (e9c7cc1) or if it is a false positive.
If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/2019/builds/205

Failed tests:

test.test_multiprocessing_fork.test_misc

Failed subtests:

test_large_pool - test.test_multiprocessing_fork.test_misc.MiscTestCase.test_large_pool

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/test/_test_multiprocessing.py", line 7306, in test_large_pool
    rc, out, err = script_helper.assert_python_ok(testfn)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 182, in assert_python_ok
    return _assert_python(True, *args, **env_vars)
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 167, in _assert_python
    res.fail(cmd_line)
    ~~~~~~~~^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 80, in fail
    raise AssertionError(f"Process return code is {exitcode}\n"
    ...<10 lines>...
                         f"---")
AssertionError: Process return code is 1
command line: ['/root/buildarea/3.15.angelico-debian-amd64/build/python', '-X', 'faulthandler', '-I', '@test_1451345_tmpæ']


Traceback (most recent call last):
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/test/_test_multiprocessing.py", line 7306, in test_large_pool
    rc, out, err = script_helper.assert_python_ok(testfn)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 182, in assert_python_ok
    return _assert_python(True, *args, **env_vars)
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 167, in _assert_python
    res.fail(cmd_line)
    ~~~~~~~~^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/test/support/script_helper.py", line 80, in fail
    raise AssertionError(f"Process return code is {exitcode}\n"
    ...<10 lines>...
                         f"---")
AssertionError: Process return code is 1
command line: ['/root/buildarea/3.15.angelico-debian-amd64/build/python', '-X', 'faulthandler', '-I', '@test_1421029_tmpæ']


Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import sys; from multiprocessing.forkserver import main; main(10, 11, ['__main__'], sys_argv=sys.argv[1:], **{'sys_path': ['/root/buildarea/3.15.angelico-debian-amd64/build/target/lib/python315.zip', '/root/buildarea/3.15.angelico-debian-amd64/build/Lib', '/root/buildarea/3.15.angelico-debian-amd64/build/build/lib.linux-x86_64-3.15'], 'main_path': '/root/buildarea/3.15.angelico-debian-amd64/build/build/test_python_1451345æ/@test_1451345_tmpæ', 'authkey_r': 13})
                                                             ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/forkserver.py", line 406, in main
    pid = os.fork()
OSError: [Errno 12] Cannot allocate memory
Traceback (most recent call last):
  File "/root/buildarea/3.15.angelico-debian-amd64/build/build/test_python_1451345æ/@test_1451345_tmpæ", line 4, in <module>
    with multiprocessing.Pool(200) as p:
         ~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/context.py", line 119, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild,
                context=self.get_context())
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 215, in __init__
    self._repopulate_pool()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 306, in _repopulate_pool
    return self._repopulate_pool_static(self._ctx, self.Process,
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
                                        self._processes,
                                        ^^^^^^^^^^^^^^^^
    ...<3 lines>...
                                        self._maxtasksperchild,
                                        ^^^^^^^^^^^^^^^^^^^^^^^
                                        self._wrap_exception)
                                        ^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 329, in _repopulate_pool_static
    w.start()
    ~~~~~~~^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
                  ~~~~~~~~~~~^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/context.py", line 309, in _Popen
    return Popen(process_obj)
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/popen_forkserver.py", line 35, in __init__
    super().__init__(process_obj)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/popen_fork.py", line 20, in __init__
    self._launch(process_obj)
    ~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/popen_forkserver.py", line 59, in _launch
    self.pid = forkserver.read_signed(self.sentinel)
               ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/forkserver.py", line 472, in read_signed
    raise EOFError('unexpected EOF')
EOFError: unexpected EOF
---


Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import sys; from multiprocessing.forkserver import main; main(10, 11, ['__main__'], sys_argv=sys.argv[1:], **{'sys_path': ['/root/buildarea/3.15.angelico-debian-amd64/build/target/lib/python315.zip', '/root/buildarea/3.15.angelico-debian-amd64/build/Lib', '/root/buildarea/3.15.angelico-debian-amd64/build/build/lib.linux-x86_64-3.15'], 'main_path': '/root/buildarea/3.15.angelico-debian-amd64/build/build/test_python_1421029æ/@test_1421029_tmpæ', 'authkey_r': 13})
                                                             ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/forkserver.py", line 406, in main
    pid = os.fork()
OSError: [Errno 12] Cannot allocate memory
Traceback (most recent call last):
  File "/root/buildarea/3.15.angelico-debian-amd64/build/build/test_python_1421029æ/@test_1421029_tmpæ", line 4, in <module>
    with multiprocessing.Pool(200) as p:
         ~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/context.py", line 119, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild,
                context=self.get_context())
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 215, in __init__
    self._repopulate_pool()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 306, in _repopulate_pool
    return self._repopulate_pool_static(self._ctx, self.Process,
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
                                        self._processes,
                                        ^^^^^^^^^^^^^^^^
    ...<3 lines>...
                                        self._maxtasksperchild,
                                        ^^^^^^^^^^^^^^^^^^^^^^^
                                        self._wrap_exception)
                                        ^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/pool.py", line 329, in _repopulate_pool_static
    w.start()
    ~~~~~~~^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
                  ~~~~~~~~~~~^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/context.py", line 309, in _Popen
    return Popen(process_obj)
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/popen_forkserver.py", line 35, in __init__
    super().__init__(process_obj)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/popen_fork.py", line 20, in __init__
    self._launch(process_obj)
    ~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/popen_forkserver.py", line 59, in _launch
    self.pid = forkserver.read_signed(self.sentinel)
               ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/multiprocessing/forkserver.py", line 472, in read_signed
    raise EOFError('unexpected EOF')
EOFError: unexpected EOF
---

encukou requested review from AA-Turner, ZeroIntensity and markshannon as code owners June 10, 2026 13:33

encukou added topic-C-API needs backport to 3.15 bugs and security fixes labels Jun 10, 2026

bedevere-app Bot added the awaiting core review label Jun 10, 2026

bedevere-app Bot mentioned this pull request Jun 10, 2026

Implement PEP 820 – PySlot: Unified slot system for the C API #149044

Closed

encukou mentioned this pull request Jun 10, 2026

gh-149044: Implement PEP 820 – PySlot: Unified slot system for the C API #149055

Merged

encukou added 5 commits June 10, 2026 16:00

Correct PyType_Spec member links

b5e1cea

Add :c:

c0105b9

Merge in the main branch

4649993

Update removed-ids.txt

0ca84f8

Merge in the main branch

1de19c5

encukou requested review from StanFromIreland and hugovk as code owners June 11, 2026 10:27

serhiy-storchaka approved these changes Jun 13, 2026

View reviewed changes

bedevere-app Bot added awaiting merge and removed awaiting core review labels Jun 13, 2026

encukou merged commit 16185e9 into python:main Jun 17, 2026
58 checks passed

bedevere-app Bot removed the awaiting merge label Jun 17, 2026

encukou deleted the tp_base_unification branch June 17, 2026 13:27

bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label Jun 17, 2026

gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases by encukou · Pull Request #151252 · python/cpython · GitHub

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gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases - #151252

gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases#151252
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encukou:tp_base_unification

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gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases by encukou · Pull Request #151252 · python/cpython · GitHub

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gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases - #151252

gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases#151252encukou merged 6 commits intopython:mainpython/cpython:mainfrom encukou:tp_base_unificationencukou/cpython:tp_base_unificationCopy head branch name to clipboard

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gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases#151252
encukou merged 6 commits into
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encukou:tp_base_unification

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