Recipe request: asyncio "spin off coroutine"
frank at chagford.com
Thu Dec 15 00:39:59 EST 2016
"Chris Angelico" wrote in message
news:CAPTjJmoFyJqYw4G_kNo5Sn=ULyjgm=KeXqrQVWUBr87EVbzAFA at mail.gmail.com...
> When you work with threads, it's pretty straight-forward to spin off
> another thread: you start it. Your current thread continues, and the
> other one runs in parallel.
> What's the most straight-forward way to do this in asyncio?
You have got the answer - asyncio.ensure_future(...)
I thought I would share something that happened just yesterday, where this
came to my rescue.
I mentioned in a recent post that I had set up a background task to process
HTTP requests for a given session, using an asyncio.Queue and an endless
I have a session.close() coroutine which has, among other steps -
await self.request_queue.put(None) # to stop the loop
await self.request_queue.join() # to ensure all requests completed
There are three places in my code that can await session.close() -
- on closing the program, close all open sessions
- on detecting that the session is no longer responding, kill the
- on receiving a message from the client indicating that it had closed
The first two worked fine, but the third one hung. I eventually found that,
because I was awaiting session.close() from within the request handler, that
request had not completed, therefore it was hanging on the 'join'.
I was stumped for a while, and then I had an 'aha' moment.
I changed 'await self.close()', to 'asyncio.ensure_future(self.close())'.
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