python - PySpark:在 RDD 中使用对象

Question

我目前正在学习 Python，并希望将其应用到 Spark 上或与 Spark 结合使用。我有这个非常简单(且无用)的脚本:

import sys
from pyspark import SparkContext

class MyClass:
    def __init__(self, value):
        self.v = str(value)

    def addValue(self, value):
        self.v += str(value)

    def getValue(self):
        return self.v

if __name__ == "__main__":
    if len(sys.argv) != 1:
        print("Usage CC")
        exit(-1)

    data = [1, 2, 3, 4, 5, 2, 5, 3, 2, 3, 7, 3, 4, 1, 4]
    sc = SparkContext(appName="WordCount")
    d = sc.parallelize(data)
    inClass = d.map(lambda input: (input, MyClass(input)))
    reduzed = inClass.reduceByKey(lambda a, b: a.addValue(b.getValue))
    print(reduzed.collect())

用

执行时

spark-submit CustomClass.py

..出现以下错误(输出缩短):

Caused by: org.apache.spark.api.python.PythonException: Traceback (most recent call last):
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/worker.py", line 111, in main
    process()
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/worker.py", line 106, in process
    serializer.dump_stream(func(split_index, iterator), outfile)
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/serializers.py", line 133, in dump_stream
    for obj in iterator:
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/rdd.py", line 1728, in add_shuffle_key
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/serializers.py", line 415, in dumps
    return pickle.dumps(obj, protocol)
PicklingError: Can't pickle __main__.MyClass: attribute lookup __main__.MyClass failed
at org.apache.spark.api.python.PythonRunner$$anon$1.read(PythonRDD.scala:166)...

给我的声明

PicklingError: Can't pickle __main__.MyClass: attribute lookup __main__.MyClass failed

似乎很重要。这意味着类实例不能被序列化，对吧？你知道如何解决这个问题吗？

感谢和问候

Answer 1

有很多问题:

• 如果将 MyClass 放在一个单独的文件中，它可以被 pickle。这是许多 Python 使用 pickle 的常见问题。这很容易通过移动 MyClass 和使用 from myclass import MyClass 来解决。通常 dill 可以解决这些问题(如 import dill as pickle)，但它在这里对我不起作用。
• 一旦这个问题得到解决，你的 reduce 就不会工作，因为调用 addValue 返回 None(不返回)，而不是 MyClass 的实例.您需要更改 addValue 以返回 self。
• 最后，lambda 需要调用 getValue，所以应该有 a.addValue(b.getValue())

一起:myclass.py

class MyClass:
    def __init__(self, value):
        self.v = str(value)

    def addValue(self, value):
        self.v += str(value)
        return self

    def getValue(self):
        return self.v

main.py

import sys
from pyspark import SparkContext
from myclass import MyClass

if __name__ == "__main__":
    if len(sys.argv) != 1:
        print("Usage CC")
        exit(-1)

    data = [1, 2, 3, 4, 5, 2, 5, 3, 2, 3, 7, 3, 4, 1, 4]
    sc = SparkContext(appName="WordCount")
    d = sc.parallelize(data)
    inClass = d.map(lambda input: (input, MyClass(input)))
    reduzed = inClass.reduceByKey(lambda a, b: a.addValue(b.getValue()))
    print(reduzed.collect())