> STINNER Victor added the comment:
>
>> I think that's a cleaner design.
>
> For common containers like Sequence or Mapping, ABC are useful. But I don't expect new implementations of BaseSelector. Is it just for purity? :-p

There are already implementations in the test suite, but the main
reason is for documentation purposes (i.e. to document clearly which
methods are available, and which are optional).
IMO, ABC make documentation more clear, and also make it easier for
third-party implementations.

I don't have a strong opinion on this, my real concern is really just
the documentation, so if you can propose a documentation patch that
makes things clear, I'm perfectly fine with removing BaseSelector from
the API :-)