Is this a false negative about Self

[lexdene]

code (src/main.py):

from typing import Self

class Foo:
    key: str
    name: str

    def __init__(self, key: str, name: str):
        self.key = key
        self.name = name

    @classmethod
    def get(cls, key: str) -> Self:
        return _data[key]

_data: dict[str, Foo] = {
    '0': Foo('0', 'hello'),
    '1': Foo('1', 'world'),
}

script:

mypy -V
mypy src

expect output:

mypy 1.2.0 (compiled: yes)
Success: no issues found in 1 source file

actual output:

mypy 1.2.0 (compiled: yes)
src/main.py:14: error: Incompatible return value type (got "Foo", expected "Self")  [return-value]
Found 1 error in 1 file (checked 1 source file)

additional infos:

$ pip show mypy
Name: mypy
Version: 1.2.0
Summary: Optional static typing for Python
Home-page: http://www.mypy-lang.org/
Author: Jukka Lehtosalo
Author-email: [email protected]
License: MIT License
Location: ~/venv/lib/python3.11/site-packages
Requires: mypy-extensions, typing-extensions
Required-by: 
$ python --version
Python 3.11.1
$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 20.04.6 LTS
Release:	20.04
Codename:	focal

[JelleZijlstra]

The error is correct. If you have a subclass of Foo, this function will still return Foo objects, not instances of the subclass.

[lexdene]

How should I tell mypy that _data[key] is an instance of Foo and will never be an instance of a subclass of Foo?

[JelleZijlstra]

Your method should be marked as returning Foo.

[deliro]

It's worth noting that marking a class as @final is also a solution (IMO a better one)