pyspark将dataframe中的none转为空数组

from pyspark.sql.functions import *
from pyspark.sql.types import *

df = spark.createDataFrame([{'a': None, 'b': 2}, {'a': [2,3], 'b': 4}])
empty_array = udf(lambda :[], ArrayType(LongType()))

# solution 1
df.withColumn('a', coalesce(col('a'), empty_array())).show()

# solution 2
df.withColumn('a', when(isnull('a'), empty_array()).otherwise(df.a)).show()