Cross Thread Async

Cross-Thread Async Patterns

Handle asyncio callbacks that fire from non-async threads — GUI event loops, mDNS service browsers, hardware polling, or thread-pool workers.

The Problem

When asyncio.get_event_loop() is called from a thread that wasn't started by asyncio.run(), Python 3.10+ raises:

RuntimeError: There is no current event loop in thread 'zeroconf-ServiceBrowser-...'

Because asyncio.run() installs the loop only on the calling thread. Sub-threads do not inherit it.

The Fix: Store the Loop Reference

class ServiceWithCallbacks:
    def __init__(self):
        self._loop = None  # Set externally

    def set_event_loop(self, loop):
        self._loop = loop

    def _on_service_state_change(self):
        # WRONG — fails in non-async threads:
        # asyncio.get_event_loop()  → RuntimeError

        # RIGHT:
        asyncio.run_coroutine_threadsafe(
            self._on_device_added(),
            self._loop  # stored reference
        )

Wire at startup:

loop = asyncio.get_running_loop()
service.set_event_loop(loop)

When This Pattern Applies

Alternative: Dedicated Async Thread

When ALL async work lives in a background thread:

def background_worker():
    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop)
    loop.run_forever()

thread = threading.Thread(target=background_worker, daemon=True)
thread.start()
# Now get_event_loop() works inside this thread

Use this for pure async-daemon patterns. Don't use it if the main thread also needs async.

Detecting the Bug Early

# Add a guard at API boundaries
import asyncio

def schedule_coro(coro):
    try:
        loop = asyncio.get_running_loop()
        # We're in an async context — schedule directly
        asyncio.ensure_future(coro)
    except RuntimeError:
        # No running loop — must have stored reference
        raise RuntimeError(
            "Called from non-async thread without stored loop reference"
        )

Source of the Pattern

Discovered while debugging the InstantTransfer project:

• zeroconf.ServiceBrowser fires ServiceStateChange callbacks from a background thread
• Calling asyncio.get_event_loop() there raises RuntimeError
• Fix: store self._loop = asyncio.get_running_loop() in the async init, use asyncio.run_coroutine_threadsafe(coro, self._loop) in callbacks