Pyspark User-Defined_functions inside of a class

Question

I am trying to create a Spark-UDF inside of a python class. Meaning, one of the methods in a class is the UDF. I am getting an error named " PicklingError: Could not serialize object: TypeError: can't pickle _MovedItems objects "

Environment : Azure Databricks . (DBR version 6.1 Beta) Code execution : In the built in Notebook. Python version : 3.5 Spark version : 2.4.4

I have tried defining the UDF outside of the class in a separate cell, and the UDF works. I do not want to write code like that, I need to follow OOP principles and would like to keep it structured. I have tried everything on Google, did not help. In fact I did not even get the information about the error I am getting. " PicklingError: Could not serialize object: TypeError: can't pickle _MovedItems objects "

class phases():
  def __init__(self, each_mp_pair_df_as_arg, unique_mp_pair_df_as_arg):
    print("Inside the constructor of Class phases ")

    #I need the below 2 variables to be used in my UDF, so i am trying to put 
    them in a class
    self.each_mp_pair_phases_df = each_mp_pair_df_as_arg
    self.unique_mp_pair_phases_df = unique_mp_pair_df_as_arg

  #This is the UDF. 
  def phases_commence(self,each_row):
    print(a)
    return 1

  #This is the function that registers the UDF, 
  def initiate_the_phases_on_the_major_track_segment(self):
    print("Inside the 'initiate_the_phases_on_the_major_track_segment()'")

    #registering the UDF
    self.phases_udf = udf(self.phases_commence,LongType())
    new_df = self.each_mp_pair_phases_df.withColumn("status", self.phases_udf((struct([self.each_mp_pair_phases_df[x] for x in self.each_mp_pair_phases_df.columns]))))
    display(new_df)

#This is a method in a different notebook that creates an object for the above shown class and calls the methods that registers the UDF.
def getting_ready_for_the_phases(each_mp_pair_df_as_arg, unique_mp_pair_df_as_arg):

  phase_obj = phases(each_mp_pair_df_as_arg, unique_mp_pair_df_as_arg)
  phase_obj.initiate_the_phases_on_the_major_track_segment()

The error message is: PicklingError: Could not serialize object: TypeError: can't pickle _MovedItems objects

Answer 1

Your function needs to be static in order to define it as an udf. I was looking for some documentation to provide a good explanation, but couldn't really find it.

Basically (maybe not 100% accurate; corrections are appreciated) when you define an udf it gets pickled and copied to each executor automatically, but you can't pickle a single method of a class which is not defined at the top level (the class is part of the top level but not its methods). Have a look at this post for workarounds other than static methods.

import pyspark.sql.functions as F
import pyspark.sql.types as T


class Phases():
  def __init__(self, df1):
    print("Inside the constructor of Class phases ")

    self.df1 = df1
    self.phases_udf = F.udf(Phases.phases_commence,T.IntegerType())

  #This is the UDF. 
  @staticmethod
  def phases_commence(age):
    age = age +3
    return age

  #This is the function that registers the UDF, 
  def doSomething(self):
    print("Inside the doSomething")
    self.df1 = self.df1.withColumn('AgeP2', self.phases_udf(F.col('Age')))

l =[(1,   10   ,  'F')
,(2 ,   2   ,  'M')
,(2 ,  10  ,   'F')
,(2 ,  3  ,    'F')
,(3 ,  10,     'M')]

columns = ['id',  'Age',  'Gender']

df=spark.createDataFrame(l, columns)

bla = Phases(df)
bla.doSomething()
bla.df1.show()

Output:

Inside the constructor of Class phases 
Inside the 'initiate_the_phases_on_the_major_track_segment()' 
+---+---+------+-----+ 
| id|Age|Gender|AgeP2| 
+---+---+------+-----+ 
|  1| 10|     F|   13| 
|  2|  2|     M|    5| 
|  2| 10|     F|   13| 
|  2|  3|     F|    6| 
|  3| 10|     M|   13| 
+---+---+------+-----+