Can't initialize frozen dataclass properties in __init__

[Stiivi]

I assume this is rather a missing feature than a bug.

The __init__() in the following code is offensive:

from dataclasses import dataclass
from typing import Optional

@dataclass(init=False, frozen=True)
class Thing:
    name: str
    key: str

    def __init__(self, name: str, key: Optional[str]=None) -> None:
        self.name = name
        self.key = key or name

The output is:

test.py:10: error: Property "name" defined in "Thing" is read-only
test.py:11: error: Property "key" defined in "Thing" is read-only

The error follows the frozen=True flag, although I would expect to be able to initialize the properties in the __init__() (or __post_init__()) function.

MyPy version: 0.711.

[ilevkivskyi]

This is not a mypy issue, the code actually fails at runtime with

dataclasses.FrozenInstanceError: cannot assign to field 'name'