wait

Learn first-to-finish with asyncio.wait.

wait

asyncio.wait awaits a group of coroutines or Tasks; it does not have to wait for all of them. It can be set to wait for the first completion, the first error, or all.

done, pending = await asyncio.wait(tasks, return_when=asyncio.FIRST_COMPLETED)

It returns two sets: done (completed) and pending (still waiting). The return_when parameter sets when to return: ALL_COMPLETED, FIRST_COMPLETED or FIRST_EXCEPTION.

gather returns results in order; wait works with Task objects and gives finer control.

done and pending sets

The done set returned by wait holds completed Tasks; pending holds the still-waiting ones. Both contain Task objects.

When it returns on first completion, you can cancel the pending Tasks or keep waiting. This is useful for timeouts and cancellation logic.

for t in pending:
    t.cancel()

This control gives flexibility that gather does not.

Exercise First completion with wait

Start with import asyncio. The function async def slow() does await asyncio.sleep(0); return 42. Inside async def main() build tasks = [asyncio.create_task(slow())]; call done, pending = await asyncio.wait(tasks). Return return list(done)[0].result(). Add print(asyncio.run(main())).

Show example solution

import asyncio

async def slow():
    await asyncio.sleep(0)
    return 42

async def main():
    tasks = [asyncio.create_task(slow())]
    done, pending = await asyncio.wait(tasks)
    return list(done)[0].result()

print(asyncio.run(main()))

Test yourself

wait return_when

done, pending = await asyncio.wait(tasks,
                                       return_when=asyncio.____)

Show answer

done, pending = await asyncio.wait(tasks,
                                       return_when=asyncio.FIRST_COMPLETED)

Match the wait concepts

asyncio.wait

Awaits Tasks
done

Completed
pending

Still waiting
FIRST_COMPLETED

Return on first
What does asyncio.wait return?
- Only a list of results
- A single Task
- Two sets: done and pending
- Nothing
Show answer

Two sets: done and pending — wait returns two sets: done and pending.

Next lesson: Queue

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