There doesn't seem to be any way of modifying the data yielded by the yield from expression without breaking the yield from "pipe". By "pipe" I mean the fact that .send() and .throw() pass to the inner generator. Useful cases are parsers as I've demonstrated in the question, transcoders, encoding and decoding the yielded values without interrupting .send() or .throw() and generally coroutines.

I believe it would be beneficial to many aspects and libraries in Python, most notably asyncio.

I couldn't think of a good syntax though, other than creating a wrapping function, lets say in itertools, that creates a class overriding send, throw and __next__ and receives the generator and associated modification functions (like suggested in one of the answers).

What do you think? Is it useful? Any suggested syntax?