> I don't see why asyncio should prevent people to experiment their own custom executor. You can imagine a custom "green executor" which inherit from Executor but uses its own black magic like greenlet.

Because asyncio and its ecosystem is built around the fact that the default executor is a ThreadPoolExecutor.

asyncio users are free to:

1. Pass any kind of executor as a first argument to `loop.run_in_executor()` -- no restrictions there.

2. Pass a subclass of ThreadPoolExecutor to `loop.set_default_executor()` -- that's how people can experiment.

Mind that (2) also covers exotic use cases like using greenlets under the hood (although I don't think it's possible), as long as they subclass ThreadPoolExecutor.

My plan so far: 

* in 3.8 add a DeprecationWarning if an executor other than ThreadPoolExecutor is set as a default one.

* during 3.8 see what kind of feedback we get.

* if all goes great, in 3.9 we'll only allow subclasses of ThreadPoolExecutor.