code: def generate_string_idx(
      df: "SparkDataFrame",
      columns: List[str],
      freq_limit: Optional[str],
      order_by_freq: bool,
  ) -> List["SparkDataFrame"]:
      return callZooFunc(
          "float",
          "generateStringIdx",
          df,
          columns,
          freq_limit,
          order_by_freq,
      )
  
imports: from bigdl.dllib.utils.file_utils import callZooFunc
  from pyspark.sql.types import IntegerType, ShortType, LongType, FloatType, DecimalType, DoubleType, BooleanType
  from pyspark.sql.functions import broadcast, udf
  from bigdl.dllib.utils.log4Error import *
  import warnings
  from bigdl.dllib.utils.log4Error import *
  from typing import TYPE_CHECKING, Any, List, Optional, Tuple, Union
label_desc: Use image node_id 3 for calling a global function with example usage: generate_string_idx(df, columns, freq_limit, order_by_freq) and returns: callZooFunc