Klunky, and maybe we could come up with a standard way to do it and include that in numpy, but I'm not sure that ABCs are the way to do it.
ABCs are *absolutely* the way to go about it. It's the only way baked into the Python language itself that allows you to register a class for purposes of `isinstance` without needing to subclass--i.e. duck-typing.
What's needed, though, is not just a single ABC. Some thought and design needs to go into segmenting the ndarray API to declare certain behaviors, just like was done for collections:
You don't just have a single ABC declaring a collection, but rather "I am a mapping" or "I am a mutable sequence". It's more of a pain for developers to properly specify things, but this is not a bad thing to actually give code some thought.