Nice! That's something I like about Haskell and Elixir: the ability to define behavior for a given case. Though it looks like that's constrained to type-based dispatch (a la Java method overloading). Is there any way to define for specific values?

If I understand the PEP right, this explanation is missing the part where it's only useful to extend abstract classes; python classes are open so you can just add a method to a concrete class to get single-dispatch if that's all you need. I don't think the example given needs generics at all.