app.py

# Start the polling, wrap in future so that we can cancel it later.
q.client.future = asyncio.ensure_future(polling_function(q))
# Future cancellation raises an error - use trycatch.
try:
    # Wait until polling finished.
    await q.client.future
    q.page['meta'].dialog = None
    q.page['meta'].notification_bar = ui.notification_bar(
        name='success_notification',
        text='Job done!',
        type='success',
        events=['dismissed']
    )
# Handle cancellation.
except asyncio.CancelledError:
    q.page['meta'].dialog.items[0].progress.caption = 'Cancelled'