Thanks i modified my code as per your suggestion and it worked perfectly Thanks again for all your inputs

dflist= spark.createDataFrame(list(a.columns), "string").toDF("Name")

dfg=dflist.filter(col('name').isin('ref_date')).count()

if dfg==1 :

  a = a.withColumn('ref_date',expr(f"CASE WHEN flag = 3 THEN '{sf_start_dt}' ELSE ref_date END"))

else:

  a = a.withColumn('ref_date',expr(f"CASE WHEN flag = 3 THEN '{sf_start_dt}' END"))