code: def _get_free_namespace(self):
      while True:
          namespace = "mars-ns-" + str(uuid.uuid4().hex)
          try:
              self._core_api.read_namespace(namespace)
          except K8SApiException as ex:
              if ex.status != 404:  # pragma: no cover
                  raise
              return namespace
  
imports: import asyncio
  import functools
  import logging
  import random
  import time
  import uuid
  from urllib.parse import urlparse
  from ...lib.aio import new_isolation, stop_isolation
  from ...services.cluster.api import WebClusterAPI
  from ...session import new_session
  from ...utils import calc_size_by_str
  from ..utils import wait_services_ready
  from .config import NamespaceConfig, RoleConfig, RoleBindingConfig, ServiceConfig, MarsSupervisorsConfig, MarsWorkersConfig
label_desc: Use image node_id 3 for calling the KubernetesCluster obj's underlying member method code with example usage: obj._get_free_namespace() and returns: namespace