Slots does not work with typing.Final dataclass attribute

[NicoHood]

Bug report

Bug description:

Hi,
I discovered a bug when using the Final attribute along with slots=True in dataclasses. The bug is more described in this stackoverflow post where someone with more knowledge has explained to me why this happens and which code line causes this issue:
https://stackoverflow.com/questions/78039353/why-does-slots-not-work-with-final-dataclass-attribute

Here is a minimal example:

#!/usr/bin/env python3

from __future__ import annotations
from dataclasses import dataclass
from typing import Final, ClassVar

@dataclass(slots=True)
class Child():
    VERSION: Final[str] = '1.0.0'

    @classmethod
    def check_version(cls, version: str) -> bool:
        return version == cls.VERSION

print(Child.check_version('1.0.0'))
# Returns False

CPython versions tested on:

3.11

Operating systems tested on:

Linux

[AlexWaygood]

Interesting -- possibly a duplicate of #89547

[sobolevn]

I don't think that this is a bug, actually. Let's "compile" what's happening into simplier python code:

@dataclass(slots=True)
class Child:
    VERSION: Final[str] = '1.0.0'

Turns into:

class Child:
    __slots__ = ('VERSION',)
    def __init__(self, VERSION) -> None:
         self.VERSION: Final[str] = '1.0.0'

Remember that all field: annotation = default statements inside a dataclass are translated into instance attributes.

So, when you access cls.VERSION you get <member 'VERSION' of 'Child' objects>. This is also correct.

If you want to use Final with class-level variables right now you have to do something like:

class Base:
    VERSION: Final[str] = '1.0.0'

@dataclass(slots=True)
class Child(Base):
    ...

I agree with @AlexWaygood that this is a duplicate of #89547 and the proper solution would be to use something like ClassVar[Final[]] for this in the future.

I am going to close this issue as a duplicate, but feel free to ask any questions you might have :)

[NicoHood]

So is ClassVar[Final[]] the correct solution or not? You wrote "in the future". So what should I do now?

[sobolevn]

Now you can do this:

class Base:
    VERSION: Final[str] = '1.0.0'

@dataclass(slots=True)
class Child(Base):
    ...

[NicoHood]

But what if the version must be in the child, as I have different child implementations?

[sobolevn]

Use VERSION: ClassVar[str] = '1.2.3' then.

[NicoHood]

But a classvar is not final, isnt it?