code: def concat_ws(sep, *cols):
      
      sc = SparkContext._active_spark_context
      return Column(sc._jvm.functions.concat_ws(sep, _to_seq(sc, cols, _to_java_column)))
docs: Concatenates multiple input string columns together into a single string column,
      using the given separator.
  
      >>> df = spark.createDataFrame([('abcd','123')], ['s', 'd'])
      >>> df.select(concat_ws('-', df.s, df.d).alias('s')).collect()
      [Row(s=u'abcd-123')]