erroneous complaint during tree walk

[cjw296]

Bug Report

@hauntsaninja suggested I raise this.

To Reproduce

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

T = TypeVar('T')

@dataclass()
class Element(Generic[T]):
    value: T
    parent: "Element[Any] | None" = None


    def walk(self) -> None:
        current: Element[Any] | None = self
        while current := current.parent:
            print(current.value)

Expected Behavior

$ pyright --version
pyright 1.1.393
$ pyright type_bug.py 
0 errors, 0 warnings, 0 informations 

Actual Behavior

$ mypy --version
mypy 1.14.1 (compiled: yes)
$ mypy type_bug.py 
type_bug.py:14: error: Item "None" of "Element[Any] | None" has no attribute "parent"  [union-attr]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.14.1
• Python version used: Python 3.12.1

[hauntsaninja]

Thanks for filing. Looking at this, I think it's just a (slightly confusing) duplicate of #2008. Old issue, but we've recently made progress towards it. Last I checked the primer for the fix was starting to look not too bad (see hauntsaninja#4)

One way to work around is to move the assignment to a separate line from the annotation:

current: Element | None
current = self

See also:

from __future__ import annotations
from typing import Any

class Element:
    value: int
    parent: Element | None

    def walk(self) -> None:
        current: Element | None
        current = self
        while current := current.parent:
            print(current.value)

[ilevkivskyi]

Yes, a duplicate of #2008