code: def _list_gcs_subdirs(self, prefix=None):
      if prefix is None:
          prefix = self.basedir
  
      iterator = self.bucket.list_blobs(prefix=prefix, delimiter="/")
      prefixes = set()
      for page in iterator.pages:
          prefixes.update(page.prefixes)
  
      return list(prefixes)
  
imports: import json
  import os
  import os.path
  import re
  from typing import Optional
  from sacred.commandline_options import cli_option
  from sacred.dependencies import get_digest
  from sacred.observers.base import RunObserver
  from sacred.serializer import flatten
  from sacred.utils import PathType
label_desc: Use image node_id 3 for calling the GoogleCloudStorageObserver obj's underlying member method code with example usage: obj._list_gcs_subdirs(prefix) and returns: list