Comment by dragonwriter
3 days ago
> That said, I'd be curious to hear a legitimate example of using it to implement "structural subtyping that you can't express with Protocols/ABCs alone".
ABCs with __subclasshook__ have been available since Python 2.6, providing a mechanism to inplement runtime-testable structural subtyping. Protocols and @runtime_checkable, which provide typechecking-time structural subtyping (Protocols) that can also be available at runtime (with @runtime_checkable) were added in Python 3.8, roughly 11 years later.
There may not be much reason to use __subclasshook__ in new code, but there's a pretty good reason it exists.
> There may not be much reason to use __subclasshook__ in new code, but there's a pretty good reason it exists.
That's quite a different claim, and makes a lot of sense. Thanks for the history!