code: def get_active_user(self, *, retval: Optional[str]) -> None:
      self._exp_calls.append("get_active_user")
      self._retvals.append(retval)
  
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 6 for calling the MockTokenStore obj's underlying member method code with example usage: obj.get_active_user() without return types