I've tried to declare two classes

@dataclass
class Base:
    __slots__ = ('x',)
    x: Any


@dataclass
class Derived(Base):
    x: int
    y: int

As long as I correctly understood PEP-557 (inheritance part), changing type of variable is possible. This code produce error:

TypeError: non-default argument 'y' follows default argument

'x' variable in Derived class has changed default from MISSING to member_descriptor and that's the reason of the exception.

This is the same reason that this fails:

class Base:
    __slots__ = ('x',)
    x = 3

with:
ValueError: 'x' in __slots__ conflicts with class variable

In the dataclasses case, the error needs to be improved, and moved to when the base class is being defined.

I don't really get your point.

@dataclass
class Base:
    __slots__ = ('x',)
    x: Any

This case is described in PEP-557 as correct, so I don't understand why you want to generate error. Also inheritance without defining slots is correct as stated in data model.

In my opinion, member_descriptor should be treated same as MISSING and everything should work correctly.

My point is that the problem is that after:

@dataclass
class Base:
    __slots__ = ('x',)
    x: Any

Base.x has a value (it's the member_descriptor for x). That's what's causing the problem that when adding a field to the derived class, it thinks you're adding a field without a default value after one that has a default value.

I agree that I could detect this specific case and allow it. My comment about the error message was inaccurate.

There's also another major problem. Because Base.x has value, __init__ is not prepared correctly (member_descriptor is passed as default).

@dataclass
class Base:
    __slots__ = ('x',)
    x: Any

Base()  # No TypeError exception

Fixing this should be quite easy, if you want I can prepare PR.

Thanks, but I'm already looking at this in the context of a different bug.

New changeset 7389fd9 by Eric V. Smith in branch 'master':
bpo-33100: Dataclasses now handles __slots__ and default values correctly. (GH-6152)
7389fd9

New changeset 3d41f48 by Eric V. Smith (Miss Islington (bot)) in branch '3.7':
bpo-33100: Dataclasses now handles __slots__ and default values correctly. (GH-6152) (GH-6153)
3d41f48