Yes, all yield-based coroutines are generators. I know that there is a backward compatible upgrade path, but this might have a huge impact on existing code.

Interestingly, I didn't know before researching this PEP that you can actually use `return` without arguments in generators before Python 3.3 (even in 2.3) and I have worked a lot with coroutines/generators.

So I'm not even against this proposal and using `return` instead of `raise StopIteration` seems the right way to exit a generator/coroutine, but there could be lots of affected users...