code: def get_token(self, user: str) -> Optional[str]:
      return cast(
          Optional[str], self._mts._match_call(("get_token", user))
      )
  
imports: import contextlib
  import os
  from typing import Any, Iterator, List, Optional, cast
  import responses
  from responses import registries
  import determined
  from determined.common.api import authentication
label_desc: Use image node_id 4 for calling the MockTokenStoreInstance obj's underlying member method code with example usage: obj.get_token(user) and returns: cast