Update app.py

app.py

@@ -287,11 +287,26 @@ async def handle_data(request):
         "history": market_state['history']
     })
 
+async def start_background(app):
+    """Correct way to start background task in aiohttp"""
+    app['kraken_task'] = asyncio.create_task(kraken_worker())
+
+async def cleanup_background(app):
+    """Correct way to cleanup background task"""
+    app['kraken_task'].cancel()
+    try:
+        await app['kraken_task']
+    except asyncio.CancelledError:
+        pass
+
 async def main():
     app = web.Application()
     app.router.add_get('/', handle_index)
     app.router.add_get('/data', handle_data)
-    app.on_startup.append(lambda a: a.__setitem__('t', asyncio.create_task(kraken_worker())))
+    
+    # Register startup and cleanup handlers safely
+    app.on_startup.append(start_background)
+    app.on_cleanup.append(cleanup_background)
     
     runner = web.AppRunner(app)
     await runner.setup()