[Python-Dev] descriptor __set_name__ and dataclasses

[Nick Coghlan]

On 27 March 2018 at 00:40, Eric V. Smith <eric at trueblade.com> wrote:
> https://bugs.python.org/issue33141 points out an interesting issue with
> dataclasses and descriptors.
>
> Given this code:
>
> from dataclasses import *
>
> class D:
>     """A descriptor class that knows its name."""
>     def __set_name__(self, owner, name):
>         self.name = name
>     def __get__(self, instance, owner):
>         if instance is not None:
>             return 1
>         return self
>
>
> @dataclass
> class C:
>     d: int = field(default=D(), init=False)
>
> C.d.name is not set, because d.__set_name__ is never called. However, in
> this case:
>
> class X:
>     d: int = D()
>
> X.d.name is set to 'd' when d.__set_name__ is called during type.__new__.
>
> The problem of course, is that in the dataclass case, when class C is
> initialized, and before the decorator is called, C.d is set to a Field()
> object, not to D(). It's only when the dataclass decorator is run that I
> change C.d from a Field to the value of D(). That means that the call to
> d.__set_name__(C, 'd') is skipped. See
> https://www.python.org/dev/peps/pep-0487/#implementation-details for details
> on how type.__new__ works.
>
> The only workaround I can think of is to emulate the part of PEP 487 where
> __set_name__ is called. I can do this from within the @dataclass decorator
> when I'm initializing C.d. I'm not sure how great this solution is, since
> it's moving the call from class creation time to class decorator time. I
> think in 99+% of cases this would be fine, but you could likely write code
> that depends on side effects of being called during type.__new__.
>
> Unless anyone has strong objections, I'm going to make the call to
> __set_name__ in the @datacalss decorator. Since this is such a niche use
> case, I don't feel strongly that it needs to be in today's beta release, but
> if possible I'll get it in. I already have the patch written. And if it does
> get in but the consensus is that it's a bad idea, we can back it out.

Would it be feasible to define `Field.__set_name__`, and have that
call `default.__set_name__` when the latter exists, and be a no-op
otherwise?

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia