Python etc / asyncio.create_task


When talking about asyncio functions, sometimes I used the word "coroutine" and sometimes "task". It's time to tell you the difference:

  • coroutine is what async function returns. It can be scheduled, switched, closed, and so on. It's quite similar to generators. In fact, await keyword is nothing more than an alias for yield from, and async is a decorator turning the function from a generator into a coroutine.
  • asyncio.Future is like "promise" in JS. It is an object that eventually will hold a coroutine result when it is available. It has done method to check if the result is available, result to get the result, and so on.
  • asyncio.Task is like if coroutine and future had a baby. This is what asyncio mostly works with. It can be scheduled, switched, canceled, and holds its result when ready.

There is a cool function asyncio.create_task that can turn a coroutine into a proper task. What's cool about it is that this task immediately gets scheduled. So, if your code later encounters await, there is a chance your task will be executed at that point.

import asyncio

async def child():
    print('started child')
    await asyncio.sleep(1)
    print('finished child')

async def main():
    print('before sleep')
    await asyncio.sleep(0)
    print('after sleep')


before sleep
started child
after sleep

What happened:

  1. When create_task is called, it is scheduled but not yet executed.
  2. When main hits await, the scheduler switches to child.
  3. When child hits await, the scheduler switches to another task, which is main
  4. When main finished, returned without waiting for child to finish. It's dead in space now.

But what if you want to make sure a scheduled task finishes before exiting? You can pass the task into good old asyncio.gather. And later we'll see some ways to wait for it with timeouts or when you don't care about the result.

task = create_task(...)
await asyncio.gather(task)