code: def generate_string_idx(df, columns, freq_limit, order_by_freq):
      return callZooFunc(
          "float",
          "generateStringIdx",
          df,
          columns,
          freq_limit,
          order_by_freq,
      )
  
imports: from bigdl.dllib.utils.file_utils import get_file_list, callZooFunc
  from bigdl.dllib.utils.utils import convert_row_to_numpy
  from bigdl.dllib.utils.common import *
  from bigdl.dllib.utils.log4Error import *
  import pyspark.sql.functions
  from typing import TYPE_CHECKING, Dict, List, Optional, Type, Union
  from numpy import float32, int32
label_desc: Use image node_id 26 for calling a global function with example usage: generate_string_idx(df, columns, freq_limit, order_by_freq) and returns: callZooFunc