gh-153772: Make abc isinstance() tolerate instances without __class__ by fedonman · Pull Request #154149 · python/cpython

fedonman · 2026-07-19T14:41:14Z

The built-in isinstance() reads an instance's __class__ with a lookup that suppresses AttributeError and falls back to the object's type, so a value whose __class__ access raises still checks cleanly:

>>> class NoClass:
...     def __getattribute__(self, name):
...         if name == "__class__":
...             raise AttributeError(name)
...         return super().__getattribute__(name)
...
>>> isinstance(NoClass(), int)
False

ABCMeta.__instancecheck__ read __class__ directly instead, so the same object leaked the AttributeError when checked against a collections.abc class:

>>> from collections.abc import Mapping
>>> isinstance(NoClass(), Mapping)
AttributeError: __class__

This makes the abstract base class machinery fall back to type(instance) when __class__ is unavailable, in both the C (Modules/_abc.c) and the pure-Python (Lib/_py_abc.py) implementations, so it matches the built-in isinstance():

>>> isinstance(NoClass(), Mapping)
False

Objects without a __class__ are unusual, but they do turn up in the wild (for example some Qt widgets, as noted in the issue).

Issue: isinstance check against collection.abc classes fails if value has no __class__ #153772

…lass__ The built-in isinstance() reads an instance's __class__ with a lookup that suppresses AttributeError and falls back to the object's type, so isinstance(obj, int) returns False for an object whose __class__ access raises. ABCMeta.__instancecheck__ read __class__ directly instead, so isinstance(obj, Mapping) leaked that AttributeError. Fall back to type(instance) when __class__ is unavailable, in both the C and the pure-Python implementations, so the abstract base classes behave like the built-in isinstance(). Such objects are unusual, but they do turn up in the wild (for example some Qt widgets).

python-cla-bot · 2026-07-19T14:41:19Z

All commit authors signed the Contributor License Agreement.

fedonman · 2026-08-28T17:28:45Z

@johnslavik @JelleZijlstra I think this can be merged.

miss-islington-app · 2026-08-29T03:08:57Z

Thanks @fedonman for the PR, and @JelleZijlstra for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

bedevere-app · 2026-08-29T03:09:12Z

GH-156541 is a backport of this pull request to the 3.15 branch.

bedevere-app · 2026-08-29T03:09:17Z

GH-156542 is a backport of this pull request to the 3.14 branch.

bedevere-app · 2026-08-29T03:09:22Z

GH-156543 is a backport of this pull request to the 3.13 branch.

…class__ (GH-154149) (#156543) gh-153772: Make abc isinstance() tolerate instances without __class__ (GH-154149) The built-in isinstance() reads an instance's __class__ with a lookup that suppresses AttributeError and falls back to the object's type, so isinstance(obj, int) returns False for an object whose __class__ access raises. ABCMeta.__instancecheck__ read __class__ directly instead, so isinstance(obj, Mapping) leaked that AttributeError. Fall back to type(instance) when __class__ is unavailable, in both the C and the pure-Python implementations, so the abstract base classes behave like the built-in isinstance(). Such objects are unusual, but they do turn up in the wild (for example some Qt widgets). (cherry picked from commit 3e245fa) Co-authored-by: Vyron Vasileiadis <hi@fedonman.com>

…class__ (GH-154149) (#156542) gh-153772: Make abc isinstance() tolerate instances without __class__ (GH-154149) The built-in isinstance() reads an instance's __class__ with a lookup that suppresses AttributeError and falls back to the object's type, so isinstance(obj, int) returns False for an object whose __class__ access raises. ABCMeta.__instancecheck__ read __class__ directly instead, so isinstance(obj, Mapping) leaked that AttributeError. Fall back to type(instance) when __class__ is unavailable, in both the C and the pure-Python implementations, so the abstract base classes behave like the built-in isinstance(). Such objects are unusual, but they do turn up in the wild (for example some Qt widgets). (cherry picked from commit 3e245fa) Co-authored-by: Vyron Vasileiadis <hi@fedonman.com>

bedevere-bot · 2026-08-29T04:16:14Z

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Arch Linux Asan 3.13 (tierless) has failed when building commit 44a4628.

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/1464/builds/1221) and take a look at the build logs.
Check if the failure is related to this commit (44a4628) 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/1464/builds/1221

Failed tests:

test_io

Failed subtests:

test_daemon_threads_shutdown_stderr_deadlock - test.test_io.CMiscIOTest.test_daemon_threads_shutdown_stderr_deadlock

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

==

Click to see traceback logs

Traceback (most recent call last):
  File "/buildbot/buildarea/3.13.pablogsal-arch-x86_64.asan/build/Lib/test/test_io.py", line 4821, in test_daemon_threads_shutdown_stderr_deadlock
    self.check_daemon_threads_shutdown_deadlock('stderr')
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/buildbot/buildarea/3.13.pablogsal-arch-x86_64.asan/build/Lib/test/test_io.py", line 4811, in check_daemon_threads_shutdown_deadlock
    self.assertFalse(err.strip('.!'))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
AssertionError: '=================================================================\n==1958593==ERROR: AddressSanitizer: heap-use-after-free on address 0x7caf3b852898 at pc 0x56125a0c5146 bp 0x7b7f395fe080 sp 0x7b7f395fe070\nREAD of size 8 at 0x7caf3b852898 thread T1\n' is not false


Traceback (most recent call last):
  File "/buildbot/buildarea/3.13.pablogsal-arch-x86_64.asan/build/Lib/test/test_io.py", line 4821, in test_daemon_threads_shutdown_stderr_deadlock
    self.check_daemon_threads_shutdown_deadlock('stderr')
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/buildbot/buildarea/3.13.pablogsal-arch-x86_64.asan/build/Lib/test/test_io.py", line 4811, in check_daemon_threads_shutdown_deadlock
    self.assertFalse(err.strip('.!'))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
AssertionError: '=================================================================\n==1945944==ERROR: AddressSanitizer: heap-use-after-free on address 0x7c53b9c52898 at pc 0x5616ac49e146 bp 0x7b23b79fe080 sp 0x7b23b79fe070\nREAD of size 8 at 0x7c53b9c52898 thread T1\n' is not false

…class__ (GH-154149) (#156541) gh-153772: Make abc isinstance() tolerate instances without __class__ (GH-154149) The built-in isinstance() reads an instance's __class__ with a lookup that suppresses AttributeError and falls back to the object's type, so isinstance(obj, int) returns False for an object whose __class__ access raises. ABCMeta.__instancecheck__ read __class__ directly instead, so isinstance(obj, Mapping) leaked that AttributeError. Fall back to type(instance) when __class__ is unavailable, in both the C and the pure-Python implementations, so the abstract base classes behave like the built-in isinstance(). Such objects are unusual, but they do turn up in the wild (for example some Qt widgets). (cherry picked from commit 3e245fa) Co-authored-by: Vyron Vasileiadis <hi@fedonman.com>

bedevere-app Bot added the awaiting review label Jul 19, 2026

bedevere-app Bot mentioned this pull request Jul 19, 2026

isinstance check against collection.abc classes fails if value has no __class__ #153772

Closed

johnslavik self-assigned this Aug 13, 2026

johnslavik self-requested a review August 13, 2026 12:13

JelleZijlstra approved these changes Aug 13, 2026

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

johnslavik approved these changes Aug 15, 2026

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JelleZijlstra added needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes needs backport to 3.15 pre-release feature fixes, bugs and security fixes labels Aug 29, 2026

JelleZijlstra merged commit 3e245fa into python:main Aug 29, 2026
66 checks passed

bedevere-app Bot removed the awaiting merge label Aug 29, 2026

bedevere-app Bot removed the needs backport to 3.15 pre-release feature fixes, bugs and security fixes label Aug 29, 2026

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Aug 29, 2026

bedevere-app Bot removed the needs backport to 3.13 bugs and security fixes label Aug 29, 2026

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gh-153772: Make abc isinstance() tolerate instances without class - #154149

gh-153772: Make abc isinstance() tolerate instances without class#154149
JelleZijlstra merged 1 commit into
python:mainfrom
fedonman:fix-gh-153772-abc-instancecheck-no-class

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