_PyObject_StoreInstanceAttribute assertion fails in setattr after __dict__.clear()

[encukou]

Bug report

Bug description:

On current main (edit: debug build),

class C:
    def __init__(self):
        self.__dict__.clear()


obj = C()
obj.foo = None

fails with:

python: Objects/dictobject.c:6704: _PyObject_StoreInstanceAttribute: Assertion 'dict->ma_values == values' failed.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Notes

Originally reported here: https://discuss.python.org/t/python-3-12-3-and-3-13-0a6-released/50601/2

Linked PRs

• GH-117750: When clearing object's dict, clear inline values but leave dict attached #117808

[encukou]

Bisected to #116115

[hroncok]

In Fedora, we see a similar problem when trying to import django.conf.urls when building Django, but it's different. Commenting here, because @encukou asked us if the debug-build assertion is the same:

Traceback (most recent call last):
  File "/usr/lib/rpm/redhat/import_all_modules.py", line 171, in <module>
    main()
    ~~~~^^
  File "/usr/lib/rpm/redhat/import_all_modules.py", line 167, in main
    import_modules(modules)
    ~~~~~~~~~~~~~~^^^^^^^^^
  File "/usr/lib/rpm/redhat/import_all_modules.py", line 100, in import_modules
    importlib.import_module(module)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/usr/lib64/python3.13/importlib/__init__.py", line 88, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 1020, in exec_module
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "/builddir/build/BUILDROOT/python-django-4.2.11-3.fc41.x86_64/usr/lib/python3.13/site-packages/django/conf/urls/__init__.py", line 1, in <module>
    from django.urls import include
  File "/builddir/build/BUILDROOT/python-django-4.2.11-3.fc41.x86_64/usr/lib/python3.13/site-packages/django/urls/__init__.py", line 1, in <module>
    from .base import (
    ...<11 lines>...
    )
  File "/builddir/build/BUILDROOT/python-django-4.2.11-3.fc41.x86_64/usr/lib/python3.13/site-packages/django/urls/base.py", line 8, in <module>
    from .exceptions import NoReverseMatch, Resolver404
  File "/builddir/build/BUILDROOT/python-django-4.2.11-3.fc41.x86_64/usr/lib/python3.13/site-packages/django/urls/exceptions.py", line 1, in <module>
    from django.http import Http404
  File "/builddir/build/BUILDROOT/python-django-4.2.11-3.fc41.x86_64/usr/lib/python3.13/site-packages/django/http/__init__.py", line 9, in <module>
    from django.http.response import (
    ...<16 lines>...
    )
  File "/builddir/build/BUILDROOT/python-django-4.2.11-3.fc41.x86_64/usr/lib/python3.13/site-packages/django/http/response.py", line 19, in <module>
    from django.core.serializers.json import DjangoJSONEncoder
  File "/builddir/build/BUILDROOT/python-django-4.2.11-3.fc41.x86_64/usr/lib/python3.13/site-packages/django/core/serializers/__init__.py", line 23, in <module>
    from django.core.serializers.base import SerializerDoesNotExist
  File "/builddir/build/BUILDROOT/python-django-4.2.11-3.fc41.x86_64/usr/lib/python3.13/site-packages/django/core/serializers/base.py", line 9, in <module>
    from django.db import models
  File "/builddir/build/BUILDROOT/python-django-4.2.11-3.fc41.x86_64/usr/lib/python3.13/site-packages/django/db/__init__.py", line 50, in <module>
    signals.request_started.connect(reset_queries)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/builddir/build/BUILDROOT/python-django-4.2.11-3.fc41.x86_64/usr/lib/python3.13/site-packages/django/dispatch/dispatcher.py", line 83, in connect
    if settings.configured and settings.DEBUG:
                               ^^^^^^^^^^^^^^
  File "/builddir/build/BUILDROOT/python-django-4.2.11-3.fc41.x86_64/usr/lib/python3.13/site-packages/django/conf/__init__.py", line 104, in __getattr__
    val = getattr(_wrapped, name)
          ~~~~~~~^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'DEBUG'
python3-debug: /builddir/build/BUILD/Python-3.13.0a6/Objects/dictobject.c:7119: _PyObject_InlineValuesConsistencyCheck: Assertion `0' failed.

[hroncok]

The Django problem can be reproduced with:

$ python3.13d -m venv venv3.13d
$ venv3.13d/bin/pip install django==5.0.4
$ venv3.13d/bin/python -c 'import django.conf.urls'
...
AttributeError: 'NoneType' object has no attribute 'DEBUG'
python: /builddir/build/BUILD/Python-3.13.0a6/Objects/dictobject.c:7119: _PyObject_InlineValuesConsistencyCheck: Assertion `0' failed.

[thmo]

Here's a small reproducer for the AttributeError case:

class A():
    pass

a = A()
vars(a).clear()
vars(a).update({'foo':'bar'})
print(vars(a))
print(a.foo)

Works with 3.12, fails with 3.13a06.

[gvanrossum]

Fixed in main, so this can be closed.