How do I specify a default value when the value is "null" in a spark dataframe?

Question

I have a data frame like the picture below.

In the case of "null" among the values of the "item_param" column, I want to replace the string'test'. How can I do it?

df = sv_df.withColumn("srv_name", col('col.srv_name'))\
      .withColumn("srv_serial", col('col.srv_serial'))\
      .withColumn("col2",explode('col.groups'))\
      .withColumn("groups_id", col('col2.group_id'))\
      .withColumn("col3", explode('col2.items'))\
      .withColumn("item_id", col('col3.item_id'))\
      .withColumn("item_param", from_json(col("col3.item_param"), MapType(StringType(), StringType())) ) \
      .withColumn("item_param", map_values(col("item_param"))[0])\
      .withColumn("item_time", col('col3.item_time'))\
      .withColumn("item_time", from_unixtime( col('col3.item_time')/10000000 - 11644473600))\
      .withColumn("item_value",col('col3.item_value'))\
      .drop("servers","col","col2", "col3")
df.show(truncate=False)
df.printSchema()

Answer 1

Use coalesce:

.withColumn("item_param", coalesce(col("item_param"), lit("someDefaultValue"))

It will apply the first column/expression which is not null