What are the advantages of asyncio over threads?

[njs (Nathaniel J. Smith)]

@akorshkov

Well, asyncio multitasking is cooperative multitasking, so I can be sure that my code will not be interrupted between await calls (which, by the way, may be disguised under async for , async with , etc.).

There’s no “etc” here :-). There are exactly three syntactic forms that can do an await, and those are await, async for, and async with.

Anyway, in Python, the three fundamental advantages of async/await over threads are:

• Cooperative multi-tasking is much lighter-weight than OS threads, so you can reasonably have millions of concurrent tasks, versus maybe a dozen or two threads at best.
• Using await makes visible where the schedule points are. This has two advantages:
  • It makes it easier to reason about data races
  • A downside of cooperative multi-tasking is that if a task doesn’t yield then it can accidentally block all other tasks from running; if schedule points were invisible it would be more difficult to debug this issue.
• Tasks can support cancellation. (Which also benefits from making await visible, because it makes it possible to reason about which points are cancellation points.)

You might not find these reasons compelling for your particular situation. That’s fine. You can still use threads if those are more appropriate. If you only want lighter-weight cooperative multitasking and don’t care about the other issues, then you can still use gevent. But those are the reasons that async/await works the way it does.