gh-102699: Add dataclasses.DataclassLike

[AlexWaygood]

Add dataclasses.DataclassLike, an abstract base class for dataclasses. Mainly useful for type-hinting. See the issue for a more complete rationale for why this class would be useful.

• Issue: Add a Dataclass abstract base class for instance checking and type annotations #102699

[sobolevn]

Why do we need this? In my opinion, def __subclasshook__(cls, other): is enough.

For example, your own tests do not use this attribute.

[AlexWaygood]

Without this attribute, the abstract base class for all dataclasses would not itself be considered a dataclass by the dataclasses internal machinery, which would be quite bizarre in my opinion:

>>> from dataclasses import *
>>> is_dataclass(DataclassLike)
True
>>> del DataclassLike.__dataclass_fields__
>>> is_dataclass(DataclassLike)
False

But you are correct that this currently isn't covered. I will add a test for this :)

[sobolevn]

But, is DataclassLike a dataclass? :)

You can have two opinions here.

[AlexWaygood]

The contract here is that "all classes for which is_dataclass evaluates to True shall be considered subclasses of DataclassLike. But DataclassLike is considered a subclass of DataclassLike (because all classes are considered subclasses of themselves in Python). So it would be very strange, in my opinion, for there to be one class (DataclassLike itself) where is_dataclass(x) evaluates to False but it's nonetheless considered a subclass of DataclassLike.

[AlexWaygood]

Alternatively, I could make DataclassLike an actual dataclass, I suppose? Rather than "faking it". What do you think?

[AlexWaygood]

Do you like it better after adad189? :)

[AlexWaygood]

The feature freeze is fast approaching now, but here's why I haven't merged this yet:

1. I don't want to do so without @ericvsmith's approval. Once this is added to dataclasses, it'll be hard to remove again.
2. I'm not 100% sure adding this to the stdlib now is appropriate. @hauntsaninja is working on a useful_types package, to be published on PyPI, that would distribute useful types that we make common use of in typeshed. Once that package has been set up, it might be better to add DataclassLike to that library and wait for a bit, instead of adding it straight to the stdlib. That way, we can see if any major bugs or usability issues crop up before we add it to the stdlib.

[AlexWaygood]

This protocol is now available in the third-party useful_types package, maintained by @hauntsaninja, @JelleZijlstra and myself. I'm closing this PR for now, until we see whether people find it useful.