Interesting problem. Since the solution is just updating
`__abstractmethods__`, this could be done in the `@dataclass` decorator, no
new ABC needed.
On Thu, Sep 24, 2020 at 8:32 AM Ben Avrahami <avrahami....@gmail.com> wrote:
> Hey all, I recently ran into some trouble and that I think deserves some
> attention. Consider the following case:
>
> class A(ABC):
> @abstractmethod
> def __lt__(self, other):
> pass
>
> @dataclass(order=True)
> class B(A):
> x: int = 0
>
> class C(B):
> pass
>
> Although B technically implements A's abstract methods, it is still
> considered an abstract class, and calling B() will fail. However, C is
> considered a non-abstract class and calling C() will succeed. This is
> because a class's "abstraction" is decided when a class is created. When B
> is created it does not implement __lt__, the implementation is added after
> creation by the dataclass decorator. Since C inherits from B after having
> its __lt__ set, C is considered non-abstract. This will also affect any
> "mixin-decorator" such as total_ordering, and also (a less-preferred use
> case) implementing abstract methods after class creation.
>
> There are many solutions for this, my proposed solution is for abc.ABCMeta
> to implement a __setattr__, that erases unimplemented metaclasses f they
> are implemented after class creation, Something along these lines:
> class ABCMeta2(ABCMeta):
> def __setattr__(self, key, value):
> super().__setattr__(key, value)
> if key != '__abstractmethods__' and value is not None:
> self.__abstractmethods__ -= {key}
>
> Obviously some additional logic is needed to handle sub-classes and such,
> but I think it's a good starting point.
>
> Any thoughts?
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