Comment by rtpg

While I wouldn't go as far as to say that this is "the point" of ABC, it's certainly relatively important, with __subclass_hook__ being promimently placed near the top of the ABC documetnation.

Control over destructuring isn't entirely new territory for PLs, Scala has Extractor Objects[0], as an example.

I think that it's a bit easy to say "it should just match the type!" when the reality is that even basic classes like list get overwritten in Python. Ultimately many language features have configurable features through dunder methods, and the fact that those get used by other language features is a feature, not a bug IMO.

As usual, don't use libraries that do weird stuff... and every once in a while you'll have the nice DSL that does something useful in this space and it will work well.

The thought experiment about a more restrictive version of this: how does Python tell that an object is a list? If it's through isinstance, then you're hooking into a bunch of tooling that have hooks that can be overwritten. If it's _not_ through isinstance, suddenly you have multiple ways to test if something is a list (which is a problem).

[0]: https://docs.scala-lang.org/tour/extractor-objects.html