So the summary.
The major advantage of asyncio approach vs green-threads approach is that with asyncio we have cooperative multitasking and places in code where some task can yield control are clearly indicated by await, async with and async for. It makes it easier to reason about common concurrency problem of data races.
It is very important, that concurrency problems are not gone completely. You can not simply ignore other concurrent tasks. With asyncio approach it is much a less common situation when you need to use mutexes to guard “critical sections” of your code. But you should understand that every await call breaks your critical section.
To disadvantages now.
asyncio multitasking is a cooperative multitasking.
Now we have two types of functions: usual and async. This is a very big feature of a language. I would say it is quite a strange feature because it is almost impossible to use it without a carefully crafted library. (Give usual python programmer an async function and ask him to execute it without any libraries - I bet most of them would need to read documentation to do it).
This feature makes language more complicated. Many library decorators now have to check what type of function is being decorated: if it is usual function let’s do this, but if it is an async function let’s do something different.
As I mentioned previously, it is not possible to call async functions from usual functions. Looks like this is a major problem. I’ve seen at least one proposal to implement workarounds to this problem, but of course any workaround will break the only advantage of asyncio approach. You will not be sure any more that your code will not switch to another task somewhere between awaits.
If you have any library with async interface you simply can’t use this library unless your application uses some asyncio framework. At least I do not know any simple way to do it. Should Sqlalchemy provide async interface? Probably not, because than they would need to provide two interfaces or force people to switch to asyncio frameworks. But what if some database requests are heavy? My coroutine would stuck waiting for results blocking all other tasks (because multitasking is cooperative). So I have to run some requests handlers in separate threads and the only potential advantage of asyncio approach disappears.
And you have to use await and async keywords all over your code even when this particular piece of code is completely concurrecncy-safe.
I can see many problems with asyncio approach and when I started this thread I sincerely hoped that there are some advantages I do not know about. Looks like there are not. One advantage, which is in my opinion not worth all the problems. Just curious, does anyone else share this opinion?
Sorry if my post was not very calm and for not very smooth english.