16.3. AsyncIO Wait

  • asyncio.wait(aws, timeout)

  • wait() - when a timeout occurs: does not cancel the futures

  • If aw is a coroutine it is automatically scheduled as a Task

  • Returns those implicitly created Task objects in (done, pending) sets

16.3.1. SetUp

>>> import asyncio

16.3.2. Wait

  • Coroutine asyncio.wait(aws, *, timeout=None, return_when=ALL_COMPLETED)

  • aws must be iterable (list, tuple, set)

  • aws iterable must not be empty

  • Run awaitable objects in the aws concurrently and block until the condition specified by return_when

  • timeout: float|int if specified, maximum number of seconds to wait before returning

  • wait() does not cancel the futures when a timeout occurs

  • If gather() is cancelled (ie. on timeout), all submitted awaitables (that have not completed yet) are also cancelled

  • return_when indicates when this function should return

  • return_when must be one of FIRST_COMPLETED, FIRST_EXCEPTION, ALL_COMPLETED

  • return_when=FIRST_COMPLETED - The function will return when any future finishes or is cancelled;

  • return_when=FIRST_EXCEPTION - The function will return when any future finishes by raising an exception. If no future raises an exception then it is equivalent to ALL_COMPLETED;

  • return_when=ALL_COMPLETED - The function will return when all futures finish or are cancelled

  • Does not raise asyncio.TimeoutError

  • Futures or Tasks that aren't done when the timeout occurs are simply returned in the second set (pending).

done, pending = await asyncio.wait(aws)

>>> async def work():
...     return 'done'
>>>
>>>
>>> async def main():
...     task = asyncio.create_task(work())
...     done, pending = await asyncio.wait({task})
...
...     if task in done:
...         print('work is done')
>>>
>>>
>>> asyncio.run(main())
work is done