Implement @dataclass for cdef classes

PEP 557 defines dataclasses, a new feature in Py3.7. It would be nice if Cython could generate equivalent code for cdef classes directly at translation time when it finds an @dataclass constructor on them.

This can be done by generating Cython code and injecting it into the module, similar to what we do for fused types (see FusedNode.py) or auto-pickling (see the AnalyseDeclarationsTransform in ParserTreeTransforms.py). The latter would probably also be a good place to implement the @dataclass decorator, although it seems worth keeping the main part of the implementation in a separate module instead of the transform itself, e.g. Cython.Compiler.Dataclasses.

Since this only applies to cdef classes, it seems better to use a safe and dedicated separate decorator @cython.dataclass than to rely on the generic Python decorator, at least initially.

[scoder]

cdef dataclasses have landed. Happy to get feedback.

[da-woods]

What is the status of this?

The status is basically:

• Regular (i.e. non-cdef) dataclasses should work in the latest alpha release, although there’s probably a few bugs you may run into if using dataclasses.Field.
• There’s a workable PR that implements cdef class dataclasses (i.e. this issue). If you’re feeling adventurous you can check out that code and use it yourself right now. Any feedback you have from trying to use it would be useful.
• The exact implementation of the PR may change, but the features probably won’t (because they’re largely fixed by the need to copy Python). Therefore any code you write to work with the PR will probably work will whatever is released.
• The PR needs further review before it can be merged. Unfortunately it’s quite a large change (and some of the code may be “mixed” quality) which makes it quite a slow process to review.
• Hopefully it might make it in the next alpha release (or the one after that). But nobody would like to make promises to give dates.