[Python-Dev] PEP 492: What is the real goal? (original) (raw)

Greg Ewing greg.ewing at canterbury.ac.nz
Thu Apr 30 07:39:58 CEST 2015

Paul Moore wrote:

I agree. While I don't use coroutines/asyncio, and I may never do so, I will say that I find Python's approach very difficult to understand.

Well, I tried to offer something easier to understand. The idea behind PEP 3152 is that writing async code should be just like writing threaded code, except that the suspension points are explicit. But apparently that was too simple, or something.

Looking at the Wikipedia article on coroutines, I see an example of how a producer/consumer process might be written with coroutines:

var q := new queue coroutine produce loop while q is not full create some new items add the items to q yield to consume coroutine consume loop while q is not empty remove some items from q use the items yield to produce

Aaargh, this is what we get for overloading the word "coroutine". The Wikipedia article is talking about a technique where coroutines yield control to other explicitly identified coroutines.

Coroutines in asyncio don't work that way; instead they just suspend themselves, and the event loop takes care of deciding which one to run next.

I can't even see how to relate that to PEP 429 syntax. I'm not allowed to use "yield",

You probably wouldn't need to explicitly yield, since you'd use an asyncio.Queue for passing data between the tasks, which takes care of suspending until data becomes available.

You would only need to yield if you were implementing some new synchronisation primitive. Yury's answer to that appears to be that you don't do it with an async def function, you create an object that implements the awaitable-object protocol directly.

-- Greg

More information about the Python-Dev mailing list