code: def index(self):
      return self._index
  
file_path: mars/mars/oscar/backends/ray/tests/test_ray_actor_driver.py
imports: import collections
  import os
  import pytest
  from .....None import oscar
  from .....tests.core import require_ray
  from .....utils import lazy_import
  from ..communication import RayServer
  from ..driver import RayActorDriver
  from ..utils import node_address_to_placement, process_placement_to_address, addresses_to_placement_group_info, placement_group_info_to_addresses, get_placement_group, process_address_to_placement
label_desc: Use image node_id 3 for calling the DummyActor obj's underlying member method code with example usage: obj.index() and returns: self