class_docstr: Router provides mapping from external address to internal address.
code: def external_address(self):
      if self._curr_external_addresses:
          return self._curr_external_addresses[0]
  
file_path: mars/mars/oscar/backends/router.py
imports: import threading
  from typing import Dict, List, Tuple, Type, Any, Optional
  from .communication import get_client_type, Client
label_desc: Use image node_id 9 for calling the Router obj's underlying member method code with example usage: obj.external_address() and returns: self