Default value for Generic self is erased by @dataclass

[zmievsa]

Note that pyright has a similar behavior so I am not sure whether it is a bug per se but I do believe that it is an incorrect behavior.

Bug Report

It is possible to specify the default value for a generic self using __init__ parameter type hint or __new__ return type hint. However, when a @dataclass decorator is applied to the class and when it generates __init__, the __new__ definition gets erased.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&gist=13b4e50085788299b532d56cf70c3eb8

Expected Behavior
Mypy must show:

main.py:22: note: Revealed type is "__main__.User[Literal['NotLoggedIn']]"

And it does show it if the @dataclass gets commented out, init=False is passed to the dataclass, or if the following __init__ is added to the class:

def __init__(self: "User[Literal['NotLoggedIn']]", name: str) -> None:
    self.name = name

Actual Behavior

main.py:21: error: Need type annotation for "f"  [var-annotated]
main.py:22: note: Revealed type is "__main__.User[Any]"

Your Environment

• Mypy version used: 0.991
• Python version used: 3.11

Relevant links

#4236

[erictraut]

FWIW, I closed this issue in the pyright issue tracker because I don't consider this a bug in pyright. The mechanism by which a constructor's specialized type is influenced by an annotated self parameter within an __init__ mechanism has never been specified or documented. If this is something that we want to officially support in the Python type system, someone should write a PEP, and we should specify and discuss how it should work in detail. Until then, I consider this a narrowly-scoped hack that works around a very specific issue.

[zmievsa]

@erictraut do I understand correctly that your work with @JelleZijlstra on PEP 696 is supposed to solve this issue in its entirety?

[erictraut]

@Ovsyanka83, no, this issue has nothing to do with PEP 696 (or PEP 695).

[zmievsa]

@erictraut I decided to go back and check whether I'll be able to use typevardefaults with the latest pyright and succeeded.

And it does solve my problem in its entirety. Take a look at the modified code that reveals the expected type:

from dataclasses import dataclass
from typing import Generic, TypeVar, cast

from typing_extensions import TYPE_CHECKING, Literal

if TYPE_CHECKING:
    T = TypeVar("T", bound=Literal["LoggedIn", "NotLoggedIn"], default=Literal["NotLoggedIn"])


@dataclass
class User(Generic[T]):
    name: str

    def log_in(self) -> "User[Literal['LoggedIn']]":
        return cast(User[Literal["LoggedIn"]], self)

    def scream(self: "User[Literal['LoggedIn']]") -> None:
        print(f"{self.name} screams!")


f = User("admin")
reveal_type(f)

I apologize for disturbing you and I am really grateful for your work on PEP 696 in pyright.

[JelleZijlstra]

(Also, to be clear, PEP 696 is @Gobot1234's work, not mine; I am just the PEP sponsor.)

[hauntsaninja]

Closing in favour of whatever issue is tracking 696