Issue
When I run the following python code:
import asyncio
import logging
logging.basicConfig(level=logging.DEBUG)
async def read_future(fut):
print(await fut)
async def write_future(fut):
fut.set_result('My Value')
async def main():
loop = asyncio.get_running_loop()
fut = loop.create_future()
asyncio.gather(read_future(fut), write_future(fut))
asyncio.run(main(), debug=True)
Instead of read_future
waiting for the result of fut
to be set, the program crashes with the following error:
DEBUG:asyncio:Using selector: KqueueSelector
ERROR:asyncio:_GatheringFuture exception was never retrieved
future: <_GatheringFuture finished exception=CancelledError() created at /Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/tasks.py:642>
source_traceback: Object created at (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/runners.py", line 43, in run
return loop.run_until_complete(main)
File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/base_events.py", line 566, in run_until_complete
self.run_forever()
File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/base_events.py", line 534, in run_forever
self._run_once()
File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/base_events.py", line 1763, in _run_once
handle._run()
File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/events.py", line 88, in _run
self._context.run(self._callback, *self._args)
File "<stdin>", line 4, in main
File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/tasks.py", line 766, in gather
outer = _GatheringFuture(children, loop=loop)
File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/tasks.py", line 642, in __init__
super().__init__(loop=loop)
concurrent.futures._base.CancelledError
DEBUG:asyncio:Close <_UnixSelectorEventLoop running=False closed=False debug=True>
What am I doing wrong in this code? I want to be able to await the Future fut
and continue after the Future has a value/exception set.
Solution
Your problem is that asyncio.gather
is itself async (returns an awaitable); by not await
ing it, you never handed control back to the event loop, nor did you store the awaitable, so it was immediately cleaned up, implicitly cancelling it, and by extension, all of the awaitables it controlled.
To fix, just make sure you await
the results of the gather
:
await asyncio.gather(read_future(fut), write_future(fut))
Answered By - ShadowRanger
0 comments:
Post a Comment
Note: Only a member of this blog may post a comment.