code: def format_number(col, d):
      
      sc = SparkContext._active_spark_context
      return Column(sc._jvm.functions.format_number(_to_java_column(col), d))
docs: Formats the number X to a format like '#,--#,--#.--', rounded to d decimal places
      with HALF_EVEN round mode, and returns the result as a string.
  
      :param col: the column name of the numeric value to be formatted
      :param d: the N decimal places
  
      >>> spark.createDataFrame([(5,)], ['a']).select(format_number('a', 4).alias('v')).collect()
      [Row(v=u'5.0000')]