creating Client outside async starts a loop?

[ofer-pd]

Hello. This might be my own ignorance - I just started learning async Python in order to use aiomqtt. (Latest version: 2.3.0)

This example demonstrates the problem:

import aiomqtt
import asyncio

async def main_async( client ):
    async with client:
        await client.subscribe( "temperature/#" )
        async for message in client.messages:
            print( f"{ message.topic }" )

client = aiomqtt.Client( "test.mosquitto.org" )
asyncio.run( main_async( client ) )

Which produces an error like: "RuntimeError: Task <Task pending name='Task-1' coro=<main_async() running at /home/ofer/src/foo.py:5> cb=[_run_until_complete_cb() at /home/ofer/.pyenv/versions/3.12.0/lib/python3.12/asyncio/base_events.py:180]> got Future attached to a different loop"

But moving the creation of the client inside the async function solves it:

import aiomqtt
import asyncio

async def main_async():
    client = aiomqtt.Client( "test.mosquitto.org" )
    async with client:
        await client.subscribe( "temperature/#" )
        async for message in client.messages:
            print( f"{ message.topic }" )

asyncio.run( main_async() )

Is this normal / to-be-expected? Might it be worth noting in the docs for noobs?

PS - I got the pattern of creating the client separate from using it from the example on how to handle reconnections: https://aiomqtt.felixboehm.dev/reconnection.html

PPS - This started because I was trying to create a class wrapper around my use of aiomqtt where the client gets created in def __init__( self ) but doesn't get used until async def run( self ) is called.

[empicano]

Hi there, thanks for the detailed issue and sorry for my late reply! 🙂

What happens is that Client.__init__ calls asyncio.get_event_loop() which returns the running event loop if it exists and the result of get_event_loop_policy().get_event_loop() when it doesn't. When we initialize the client before asyncio.run in your example, there is no running event loop yet, and we thus fail later on.

asyncio.get_event_loop() is deprecated since Python 3.12, so I've changed it to asyncio.get_running_loop(). This means that your first example will fail directly when initializing the Client. This is similar to how e.g. aiohttp handles it.

---

For reference, your first example will now throw RuntimeError: no running event loop. For anyone stumbling upon this issue, the solution is to put the client initialization somewhere inside async context.

Hope that helps!