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Python has had async for 10 years – why isn't it more popular?

(tonybaloney.github.io)
320 points willm | 3 comments | 02 Sep 25 17:24 UTC | HN request time: 0.012s | source
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xg15 ◴[02 Sep 25 18:40 UTC] No.45107259[source]▶
I learned about the concept of async/await from JS and back then was really amazed by the elegance of it.

By now, the downsides are well-known, but I think Python's implementation did a few things that made it particularly unpleasant to use.

There is the usual "colored functions" problem. Python has that too, but on steroids: There are sync and async functions, but then some of the sync functions can only be called from an async function, because they expect an event loop to be present, while others must not be called from an async function because they block the thread or take a lot of CPU to run or just refuse to run if an event loop is detected. That makes at least four colors.

The API has the same complexity: In JS, there are 3 primitives that you interact with in code: Sync functions, async functions and promises. (Understanding the event loop is needed to reason about the program, but it's never visible in the code).

Whereas Python has: Generators, Coroutines, Awaitables, Futures, Tasks, Event Loops, AsyncIterators and probably a few more.

All that for not much benefit in everyday situations. One of the biggest advantages of async/await was "fearless concurrency": The guarantee that your variables can only change at well-defined await points, and can only change "atomically". However, python can't actually give the first guarantee, because threaded code may run in parallel to your async code. The second guarantee already comes for free in all Python code, thanks to the GIL - you don't need async for that.

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mcdeltat ◴[02 Sep 25 20:54 UTC] No.45108908[source]▶
I think Python async is pretty cool - much nicer than threading or multiprocessing - yet has a few annoying rough edges like you say. Some specific issues I run into every time:

Function colours can get pretty verbose when you want to write functional wrappers. You can end up writing nearly the exact same code twice because one needs to be async to handle an async function argument, even if the real functionality of the wrapper isn't async.

Coroutines vs futures vs tasks are odd. More than is pleasant, you have one but need the other for an API for no intuitive reason. Some waiting functions work on some types and not on others. But you can usually easily convert between them - so why make a distinction in the first place?

I think if you create a task but don't await it (which is plausible in a server type scenario), it's not guaranteed to run because of garbage collection or something. That's weird. Such behaviour should be obviously defined in the API.

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1. xg15 ◴[02 Sep 25 21:38 UTC] No.45109414[source]▶
I think the general idea of function colors has some merit - when done right, it's a crude way to communicate information about a function's expected runtime in a way that can be enforced by the environment: A sync function is expected to run short enough that it's not user-perceptible, whereas an async function can run for an arbitrary amount of time. In "exchange", you get tools to manage the async function while it runs. If a sync function runs too long (on the event loop) this can be detected and flagged as an error.

Maybe a useful approach for a language would be to make "colors" a first-class part of the type system and support them in generics, etc.

Or go a step further and add full-fledged time complexity tracking to the type system.

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2. munificent ◴[02 Sep 25 23:21 UTC] No.45110384[source]▶
> Maybe a useful approach for a language would be to make "colors" a first-class part of the type system and support them in generics, etc.

Rust has been trying to do that with "keyword generics": https://blog.rust-lang.org/inside-rust/2023/02/23/keyword-ge...

3. lmm ◴[03 Sep 25 02:30 UTC] No.45111685[source]▶
> Maybe a useful approach for a language would be to make "colors" a first-class part of the type system and support them in generics, etc.

This is what languages with higher-kinded types do and it's glorious. In Scala you write your code in terms of a generic monad and then you can reuse it for sync or async.