code: def test_async_tornado_app():
      AsyncTornadoApp.launch_instance([])
      app = AsyncApp.instance()
      assert app._prefer_selector_loop is True
      AsyncTornadoApp.clear_instance()
  
imports: from __future__ import annotations
  import asyncio
  import os
  import shutil
  from tempfile import mkdtemp
  from unittest.mock import patch
  import pytest
  from traitlets import Integer
  from jupyter_core.application import JupyterApp, JupyterAsyncApp, NoStart
  from jupyter_core.utils import ensure_event_loop
label_desc: Use image node_id 13 for calling a global function with example usage: test_async_tornado_app() without return types