As somebody who missed the discussion about it and right now took a quick look at the PEP, i ask myself how subclasses are handled, as i don't see anything about it in the PEP, just support for ABCs.
E.g if
issubclass(Apple, Fruit)
And i call a function which has registered an implementation for the Fruits type with an object of type Apple, is this implementation used? I assume so, but as said, i don't see it mentioned anywhere.
Yes, this is the supported behaviour. The PEP briefly explains that "Where there is no registered implementation for a specific type, its
method resolution order is used to find a more generic implementation."
As Nick noted, the reason the description is so brief is that this works just like calling methods.