patch does not correctly restore state on objects without __dict__

Bug report

Bug description:

from unittest.mock import patch
class Proxy(object):
    config = {}
    def __getattr__(self, name):
        return self.config[name]
    def __setattr__(self, name, value):
        if name == 'config':
            object.__setattr__(self, name, value)
            return
        self.config[name] = value
    def __delattr__(self, name):
        self.config[name] = 'DEFAULT'

p = Proxy()
p.a = True
print(p.a)
with patch('__main__.p.a', False):
    print(p.a)
print(p.a)

Expected

True
False
True

Actual

True
False
DEFAULT

Note that this is a relatively niche issue, but for example in a config system (i.e. https://github.com/pytorch/pytorch/pull/140779), if you don't want to have a delete work, (i.e. have it set it back to a default value), the code in https://github.com/python/cpython/blob/94a7a4e22fb8f567090514785c69e65298acca42/Lib/unittest/mock.py#L1637 will call delattr, and then not call setattr, because it still has the attr.

Possible fixes:
If delattr throws (maybe NotImplementederror), call setattr instead (this is nice and explicit + removes the hacky workaround I have to do).

If the value after it's been deleted doesn't equal the original value, reassign it? (this might require objects to be comparable, not sure if that is already an issue here).

Either way I wanted to see if there was any interest in fixing this, and if anyone saw a solution that seemed reasonable enough.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Commencez par la reproduction dans l’issue et examinez la logique de restauration dans Lib/unittest/mock.py autour de la ligne 1637. Comparez la manière dont patch gère les attributs des objets dépourvus de dict lorsque leur suppression modifie la valeur observée. Le travail est terminé lorsque l’exemple restaure p.a à True tout en préservant le comportement existant pour les objets ordinaires, avec l’ajout d’une couverture de régression dans les tests unittest.mock concernés.