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.

>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.

Except then your method shares a namespace with all the other methods defined on the class, which is ugly, and can cause clashes.

I had a hard time understanding the value of the idea from this as well. The link I gave made it a bit clearer to me.

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 read this article properly, there's nothing yet standardized, but there's a possibility of future development: http://lukasz.langa.pl/8/single-dispatch-generic-functions/.