GeezerVison

Well.. that's weird. The docs still fail?

Thank you. I noticed this error yesterday (this is is the second time I make such error in a month), but it was already late night.

No worry! don't hesitate to ping in the future if you encounter other Sphinx errors!

The only Py_TPFLAGS_ITEMS_AT_END type in stdlib is type itself.
So, this PR allows slotted types. Adding data descriptors to types leads to some rather hard-to-explain behaviour, for example:

>>> class Meta(type):
...     __slots__ = ['foo']  # now possible
...     
>>> class X(metaclass=Meta):
...     def foo(self): return 'f'
...     
>>> X().foo
<bound method X.foo of <__main__.X object at 0x7f7c579c2900>>
>>> X.foo
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    X.foo
AttributeError: 'Meta' object has no attribute 'foo'
>>> X.foo = slice
>>> X.foo(X())
slice(None, <__main__.X object at 0x7f7c5788c050>, None)
>>> X().foo()
'f'

Do we want to make this easier?

I did not have any use case for this feature, but this looks interesting. We already have data descriptors (__module__, __qualname__, etc). And we can get the same with properties:

class Meta(type):
    @property
    def foo(self):
        return slice

class X(metaclass=Meta):
    def foo(self): return 'f'
 
print(X.foo(X()))
print(X().foo())

Yes, descriptors are already weird. But given stdlib's non-usage of Py_TPFLAGS_ITEMS_AT_END, this feature looks to be specifically about allowing descriptors for type, so I'm not sure we want it.

What about explicitly disabling arbitrary __slots__ on type subclasses, given how many optimizations assume that the __dict__ is authoritative?

The __dict__ & __weakref__ change looks uncontroversial -- and easier to explain, too.

Was not it the same with PEP 697? It allowed extending type in C. This PR allows to extend it in Python.

Anyway, I am only interesting in extending tuples. Adding support for special slots would be a side effect. Supporting __slots__ in the type subclasses is not necessary, it would only look inconsistent to not support it after PEP 697.

See #141755.

PEP 697's “extending” was about adding a C-level chunk of memory, not about Python attributes. You can already extend type in Python. It has a __dict__.

Having a __dict__ also means the memory savings from __slots__ will be tiny, unless you have many slots. (Not to mention that type itself is huge.)

The other half of this PR (very simple) has been moved to #141764.

Lets discuss this not in PRs, but in the issue #103740.