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0 pointsscuff3d2mo ago0 comments

It absolutely does not create green threads. Green threads can be preempted and switched by the runtime. Go does this for example.

Python's asyncio is tasked based, the event loop cannot switch out a task until it reaches a yield point.

Or in short, green threads like Go uses are preemptive multitasking, the task based model asyncio uses is cooperative. A CPU bound python task can block the event loop forever if it never yields, goroutines generally can't.

It's not a philosophical question at all. A single python program can use the multiprocessing library to run multiple chunks of work in parallel. It's heavier weight thanks to the need to basically run a full python interpreter in an os process, but it provides the functionality. And the fact that it's scheduled by the OS is irrelevant. Plenty of languages use the underlying OS threading capabilities to manage threads instead of their own runtime. Both Java and C# for example (though I think Java is adding green threads now).

You're conflating several different distinct ideas. It's a common mistake I see at work all the time. Took a good amount of reading for me to untangle them all.

Edit: I left out stackless coroutines vs stackful thread like runtime. That would be more accurate then the preemptive vs cooperative stuff, but either way the gist of my comment is correct.

0 comments

TZubiri2mo ago

>the task based model asyncio uses is cooperative. A CPU bound python task can block the event loop forever if it never yields, goroutines generally can't.

I think I was wrong then. The difference between:

a = threading.Thread(request,url1).start()

b = threading.Thread(request,url2).start()

a.join()

b.join()

With similar asyncio code is that async code has to explicitly signal when the task is allowed to switch. So async is used for when greater control is required over when the scheduler can work on the two different tasks, presumably to avoid race conditions. It would be used in cases similar to where semaphores would have been used.

I do still think that it's unpythonic in that whatever you can do with async you can do without, (two ways to do things), and most of the usecases of this will be people coming from node, and people who don't know of more basic concurrent techniques.

I'm looking at the Original Article again, and it just looks like they are implementing a more complex pub-sub control flow system instead of using if statements (because that's too boring?). The traditional solution would use a socket which is essentially a thread, and the states of the connection would just be managed by TCP instead of recreated at the application layer.

I just can't shake the notion that the vast majority of cases async code in python is bad code. Of course the devs are not idiots, but my thesis is that they are bored and have to implement something, and it has an intended use case, but the bulk of usage will be for the 'wrong' reasons.

>You're conflating several different distinct ideas. It's a common mistake I see at work all the time. Took a good amount of reading for me to untangle them all.

If you have a source material to recommend on the subject (assuming I am already aware on traditional OS scheduling of processes, threads and green threads) I would be interested in reading that. It seems that even if my thesis is True, I need to understand for what precise usecases BDFL and company are developing this async thing into the language itself.

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