Repository navigation
Frozen dataclasses should have an implicit Final annotation on its members #20087
Description
Bug Report
Frozen dataclasses should be implicitly Final on its members.
Currently mypy detects correctly that a frozen dataclass member cannot be assigned to; this is similar (if not altogether equivalent) to having a Final annotation on that (and every) member. This is important because it affects type covariance:
To Reproduce
from typing import cast
from dataclasses import dataclass
class C: pass
class D(C): pass
@dataclass(frozen=True)
class MyData[T: C]:
m: T
def f(x: MyData[C]) -> None: pass
f(cast(MyData[D], 0)) # error, T is invariant here because `m` is public and deemed assignableHowever, since MyData above is frozen, m cannot be assigned to and can be deemed final for purposes of covariance.
Expected Behavior
The above should typecheck:
f(cast(MyData[D], 0)) # this should typecheckActual Behavior
The above doesn't typecheck.
Your Environment
- Mypy version used: 1.18.2
- Mypy command-line flags: nothing special
- Mypy configuration options from
mypy.ini(and other config files): strict mode = True - Python version used: 3.13.7
Activity
alecov commentedon Oct 18, 2025
The current workaround is to use an explicit
TypeVarwithcovariance=Truein cases where covariance of this kind is desirable. There are no consequences other than having uglier code. Also, note that currently there is no way to explicitly annotate members asFinal, because mypy does not accept typevars as arguments toFinal.erictraut commentedon Oct 19, 2025
This issue is discussed here. The invariance is due to the synthesized
__replace__method, which was introduced in Python 3.13.alecov commentedon Oct 20, 2025
Good, I think we can close this as a duplicate, then.