TypeError: 'str' object is not callable -Pyspark

Question

df1=df.withColumn('etl_load_dt_part_new', concat_ws("-",year(df.ETL_LOAD_DT_PART),lit('12'),lit('31')).cast('date') ) 

i am trying to add new column named as etl_load_dt_part_new , when i execute this through pyspark shell it does not throw any error , but while running through python script it gives this type error 'str' object is not callable any idea why?