asyncio event wait function does not continue after event has been stored

Question

I'm working on a function that should continue after a response has been given through a websocket. In order to achieve this I use an asyncio event which will be set after the response has been given.

Three functions are involved:

async def send(self, message): await self.channel(message.toJSON()) if (message.method == 'get' or message.method == 'post'): event = asyncio.Event() self._queueMessage(message, event) await event.wait() print('continue') def _queueMessage(self, message, event): self.queue.append([message, event]) def _process_response_message(self, message): for entry in self.queue: if (message['_id'] == entry[0]._id): print(entry[1]) entry[1].set() print(entry[1]) return

Returns:

<asyncio.locks.Event object at 0x7f3a1ff20da0 [unset,waiters:1]> <asyncio.locks.Event object at 0x7f3a1ff20da0 [set,waiters:1]>

In this example, the print('continue') function is never called and I do not why because the .set() function is actually called and .set() does seem to work fine if I use it before I call await event.wait().

Is there something I'm missing?

Are you sure control would be returned to asyncio's event loop after _process_response_message finished? — Mikhail Gerasimov
– Mikhail Gerasimov, Commented Dec 6, 2017 at 13:01
@MikhailGerasimov I assume that something like that is going wrong. I assumed that the event.set() would communicate the change to the asyncio's event loop but you are saying that is not necessarily the case? — Rick Vink
– Rick Vink, Commented Dec 6, 2017 at 13:46
@MikhailGerasimov I just added the line asyncio.get_event_loop() to the _process_response_message function and I got the message: There is no current event loop in thread 'Dummy-1'. So I think you are indeed correct with your question. — Rick Vink
– Rick Vink, Commented Dec 6, 2017 at 14:00

Mikhail Gerasimov · Accepted Answer · 2017-12-06 14:36:00Z

Based on message you get _process_response_message seems to be running in another thread. asyncio.Event is not thread-safe object you should use loop.call_soon_threadsafe function to call it's methods from other thread. Try to change your code like this:

async def send(self, message): await self.channel(message.toJSON()) if (message.method == 'get' or message.method == 'post'): loop = asyncio.get_event_loop() event = asyncio.Event() self._queueMessage(message, loop, event) await event.wait() print('continue') def _queueMessage(self, message, loop, event): self.queue.append([message, loop, event]) def _process_response_message(self, message): for entry in self.queue: qmsg, loop, event = entry if (message['_id'] == qmsg._id): loop.call_soon_threadsafe(event.set) return

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