Hybrid behavior of NG APIs

[hynek]

Originally posted by @euresti in #668 (comment)

I'm working on the mypy plugin but I am gonna need to know the final answer on how "hybrid" mode wants to work if there are both annotations and attribs. I think the options are:

1. Take only the ones that have = attr.ib()
2. Take only the annotated ones.
3. Take both.
4. Raise an exception.

Hmm. I thought #3 would be easy to implement (just don't raise the error) but it turns out we can't tell the order between annotated and un-annotated attributes.

class A:
   x: int
   y = attr.ib()

cls.__annotations__  ->   [x]
cls.__dict__.keys(). -> [y]

But which is actually first?

#2 is kind of a weird one. Why throw away those attr.ib()s?
#1 is relatively easy to implement. Because it's just a try catch.
#4 Might require the user to add ClassVar in order to add it.

Oh I should note that I think this should still be fine:

@define
class A:
   x: int = field()
   y = field()

We shouldn't force you to have to annotate everything.
Hmm. That might make 4 harder to implement. :)

[hynek]

Given your explanations and my older experiments I think only 1 and 4 are feasible.

But given your example, 4 seems also more trouble and edge cases than necessary?

I kinda still feel that the sharp edges of the hybrid approach are the best trade off because it's almost impossible to get a silent failure in a real-world project. Sure maybe you get confused the first time you get missing attributes but is it worth to force people pass arguments everywhere for it? 🤔 (skip time 1 year ahead: Hynek hates himself for this thinking 🤪)

[euresti]

FYI: I've implemented #1 for mypy. Here's the PR. It hasn't landed yet.

python/mypy#9396

[hynek]

(I'm keeping this open as long as the NG APIs are provisional, but I've mostly made up my mind. I'm happy to change it if substantial user complaints come in.)