Possible shared-keys dict bug with string subclasses

Bug report

Bug description:

So I'm not entirely sure whether this is a bug or just undefined behaviour, because I wasn't intending to do it. An unexpected code path led to some sqlalchemy objects being used as attribute names in a class without my knowledge, which was working.. until in one window I had 3.14.2 running and suddenly got an AttributeError I hadn't seen before.

Tracking it down, it turned out to be that certain attributes were appearing and disappearing in very unexpected ways.

Reducing it to the minimum case I can find, the code

import sys

print(sys.version)

class StrSub(str):
    pass

class Widget:
    pass

a = Widget()
a.date = "a_value"

b = Widget()
setattr(b, StrSub("date"), "b_value")

print(f'("date" in b.__dict__):               {("date" in b.__dict__)!s:>5}')
print(f'("date" in b.__dict__.keys()):        {("date" in b.__dict__.keys())!s:>5}')
print(f'("date" in list(b.__dict__.keys())):  {("date" in list(b.__dict__.keys()))!s:>5}')
print(f'len(b.__dict__):                      {len(b.__dict__)!s:>5}')

behaves differently in 3.14.1 and 3.14.2 than most other versions:

3.12.12 (main, Feb  3 2026, 22:51:04) [Clang 21.1.4 ]
("date" in b.__dict__):                True
("date" in b.__dict__.keys()):         True
("date" in list(b.__dict__.keys())):   True
len(b.__dict__):                          1
 
3.13.12 (main, Feb  3 2026, 22:52:12) [Clang 21.1.4 ]
("date" in b.__dict__):                True
("date" in b.__dict__.keys()):         True
("date" in list(b.__dict__.keys())):   True
len(b.__dict__):                          1
 
3.14.0 (main, Nov 19 2025, 22:48:15) [Clang 21.1.4 ]
("date" in b.__dict__):                True
("date" in b.__dict__.keys()):         True
("date" in list(b.__dict__.keys())):   True
len(b.__dict__):                          1
 
3.14.1 (main, Dec  2 2025, 19:46:13) [Clang 21.1.4 ]
("date" in b.__dict__):               False
("date" in b.__dict__.keys()):        False
("date" in list(b.__dict__.keys())):  False
len(b.__dict__):                          0
 
3.14.2 (main, Jan 27 2026, 23:59:57) [Clang 21.1.4 ]
("date" in b.__dict__):                True
("date" in b.__dict__.keys()):         True
("date" in list(b.__dict__.keys())):  False
len(b.__dict__):                          0
 
3.15.0a5 (main, Feb  3 2026, 22:53:11) [Clang 21.1.4 ]
("date" in b.__dict__):                True
("date" in b.__dict__.keys()):         True
("date" in list(b.__dict__.keys())):   True
len(b.__dict__):                          1

This is deeper in the weeds than I know much about, though!

If I convert to a plain str, everything behaves as expected.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Start by running the provided minimal reproduction across the listed Python versions and compare the dict, keys-view, and list-of-keys results. Trace the CPython dictionary handling involved in string subclasses; done means the behavior is understood and corrected or explicitly resolved with a regression test.