> and i wonder why they would It's easy, actually:

    import asyncio

    @asyncio.async  # SyntaxError in 3.7
    def my_coroutine(...): ...

And since asyncio is one of the strongest reason to migrate to python 3.x, I bet far less than 99.99% of code written for 3.4/3.5 can be run on 3.7 with no changes.

Luckly, 99% of changes were trivial. Other 1% is a nightmare.