frozen dataclass inheritance is not strictly checked in multiple inheritance

[Batman-q96]

Bug report

Bug description:

import dataclasses

@dataclasses.dataclass
class NotFrozen:
    pass

@dataclasses.dataclass(frozen=True)
class Frozen:
    pass

@dataclasses.dataclass(frozen=True)
class Child(NotFrozen, Frozen):
    pass

The dataclass inheritance hierarchy is supposed to require all classes to be either frozen or non frozen, this works properly for checking that an unfrozen class does not inherit from any frozen classes, but it allows frozen classes to inherit from unfrozen ones as long as there's at least one frozen class in the MI

CPython versions tested on:

3.10

Operating systems tested on:

Windows

Linked PRs

• gh-109409: Fix inheritance of frozen dataclass from non-frozen dataclass mixins #109437

[ericvsmith]

Are you saying Child is not frozen but you think it should be? Or the other way around? Can you show some example code that shows the problem? Like create a Child and show it is either frozen or not. Thanks.

[Batman-q96]

import dataclasses

@dataclasses.dataclass
class NotFrozen:
    pass

@dataclasses.dataclass(frozen=True)
class Frozen:
    pass

# this class fails to create with
# TypeError: cannot inherit non-frozen dataclass from a frozen one
@dataclasses.dataclass
class NotFrozenChild(NotFrozen, Frozen):
    pass

# this class has no issue with creation
@dataclasses.dataclass(frozen=True)
class FrozenChild(NotFrozen, Frozen):
    pass

The expected behavior would be that FrozenChild should fail to create with a TypeError: cannot inherit frozen dataclass from a non-frozen one. Instead FrozenChild creates with no issues (though linters such as pylance do catch this issue).

[sobolevn]

Yes, I also belive that this is a bug, here's how users can face it:

def mutate_not_frozen(instance: NotFrozen) -> None:
    x.some_field = 1

mutate_not_frozen(NotFrozen())  # ok
mutate_not_frozen(FrozenChild())  # type-checkers will allow it, but will fail in runtime

Interesting, it should fail here:

        # Raise an exception if we're frozen, but none of our bases are.
        if not any_frozen_base and frozen:
            raise TypeError('cannot inherit frozen dataclass from a '
                            'non-frozen one')

and it fails correctly with just a single parent, but multiple parents should be checked with all, not any.

[ericvsmith]

Since this is a change in behavior, I'm inclined to call it a feature request and not backport it.

[ericvsmith]

Thanks, @sobolevn !