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

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.

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.

> 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.

I think that use case doesn't work well in async, because async effectively creates a tree of Promises that resolve in order. A task that doesn't get await-ed is effectively outside it's own tree of Promises because it may outlive the Promise it is a child of.

I think the solution would be something like Linux's zombie process reaping, and I can see how the devs prefer just not running those tasks to dealing with that mess.

No, Python's system is more complex and unfortunately overloads "await" to do several things.

If you just do

  async def myAsyncFunction():
    ...
    await someOtherAsyncFunction()
    ...

then the call to someOtherAsyncFunction will not spawn any kind of task or delegate to the event loop at all - it will just execute someOtherAsyncFunction() within the task and event loop iteration that myAsyncFunction() is already running in. This is a major difference from JS.

If you just did

  someOtherAsyncFunction()

without await, this would be a fire-and-forget call in JS, but in Python, it doesn't do anything. The statement creates a coroutine object for the someOtherAsyncFunction() call, but doesn't actually execute the call and instead just throws the object away again.

I think this is what triggers the "coroutine is not awaited" warning: It's not complaining about fire-and-forget being bad style, it's warning that your code probably doesn't do what you think it does.

The same pitfall is running things concurrently. In JS, you'd do:

  task1 = asyncFunc1();
  task2 = asyncFunc2();
  await task1;
  await task2;

In Python, the functions will be run sequentially, in the await lines, not in the lines with the function calls.

To actually run things in parallel, you have to to

  loop.create_task(asyncFunc())

or one of the related methods. The method will schedule a new task and return a future that you can await on, but don't have to. But that "await" would work completely differently from the previous awaits internally.