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0 pointsxg159mo ago0 comments

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.

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2 comments · 1 top-level

everforward9mo ago· 1 in thread

I think this is semantically the same thing, though I'm sure your terminology is more correct (not an expert here).

If you do `someOtherAsyncFunction()` without await and Python tried to execute similarly to a version with `await`, then the one without await would happen in the same task and event loop iteration but there's no guarantee that it's done by the time the outer function is. Thus the existing task/event loop iteration has to be kept alive or the non-await'ed task needs to be reaped to some other task/event loop iteration.

> loop.create_task(asyncFunc())

This sort of intuitively makes sense to me because you're creating a new "context" of sorts directly within the event loop. It's similar-ish to creating daemons as children of PID 1 rather than children of more-ephemeral random PIDs.

xg15OP9mo ago

> but there's no guarantee that it's done by the time the outer function is.

As far as I understood it, calling an async function without await (or create_task()) does not run the function at all - there is no uncertainty involved.

Async functions work sort of like generators in that the () operator just creates a temporary object to store the parameters. The 'await' or create_task() are the things that actually execute the function - the first immediately runs it in the same task as the containing function, the second creates a new task and puts that in the event queue for later execution.

  asyncFunc()

without anything else is a no-op. It creates the object for parameter storage ("coroutine object") and then throws it away, but never actually calls (or schedules) asyncFunc.

When queuing the function in a new task with create_task(), then you're right - there is no guarantee the function would finish, or even would have started before the outer function completed. But the new task won't have any relationship to the task of the outer function at all, except if the outer function explicitly chooses to wait for the other task, using the Future object that was returned by create_task.

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