Bare Final special form with dataclasses

[Viicos]

Using a bare Final special form with a default value, pyright will not assume it is a normal field (probably because it assumes it is a ClassVar?):

Code sample in pyright playground

from typing import Final

from dataclasses import dataclass

@dataclass
class A:
    a: Final = 1
    b: Final[int] = 1


reveal_type(A.__init__)
# "(self: A, b: int = 1) -> None"

However, mypy seems to include a as a field (playground).

Seems like the conformance tests don't cover this use case, so it might have been an oversight.

[LeeeeT]

I wonder why python even includes a Final field into the __init__ 🤔

[Viicos]

This was clarified in the typing specification here. You can read more about the semantics of the Final special form here. While class variables are not included in the __init__ (because they shouldn't change per instance), Final (in the context of a class) means the value can't change after the class have been instantiated.

[vtgn]

I wonder why python even includes a Final field into the __init__ 🤔

In Java for example, you can declare an attribute as Final and define it either directly on the same line, or else in the constructor. Defining the value in the constructor is very useful to be able to set the final attribute with a calculated value. It's cool Python chose to do the same, but the linters are not uptodate yet.