Incorrect `__qualname__` in slotted dataclasses in `__init_subclass__`

[thejcannon]

Bug report

Bug description:

In the following:

import dataclasses

@dataclasses.dataclass(slots=True)
class Base:
    def __init_subclass__(cls) -> None:
        print(cls.__qualname__)


class Container:
    @dataclasses.dataclass(slots=True)
    class Dataclass(Base):
        pass

I get:

Container.Dataclass
Dataclass

(I can handle the double-call, but the second call having incorrect __qualname__ is just really annoying 😂 )

---

To clarify the annoyance, the real world use case looks something like:

@dataclasses.dataclass(slots=True)
class Base:
    def __init_subclass__(cls) -> None:
        if "__slots__" not in cls.__dict__:
            return  # We're the unslotted class, wait until we're the slotted "real" one
        
        ... # Some code here relying on `__qualname__`

CPython versions tested on:

3.11

Operating systems tested on:

macOS

Linked PRs

• gh-129915: Add __qualname__ to class namespace when constructing slotted dataclass #129916

[thejcannon]

(Note I tested on 3.11, because thats all I have available, but am reasonably certain the bug exists on 3.13/main)

It looks like the issue is here:

cpython/Lib/dataclasses.py

Line 1302 in 91d9544

# And finally create the class.

with the proper fix looking something like:

    # And finally create the class.
    qualname = getattr(cls, '__qualname__', None)
    if qualname is not None:
        cls_dict["__qualname__"] = qualname
    newcls = type(cls)(cls.__name__, cls.__bases__, cls_dict)

I'll get working on a branch ASAP