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I was trying to print total elements in each partitions in a DataFrame using spark 2.2

from pyspark.sql.functions import *
from pyspark.sql import SparkSession

def count_elements(splitIndex, iterator):
    n = sum(1 for _ in iterator)
    yield (splitIndex, n)

spark = SparkSession.builder.appName("tmp").getOrCreate()
num_parts = 3
df = spark.read.json("/tmp/tmp/gon_s.json").repartition(num_parts)
print("df has partitions."+ str(df.rdd.getNumPartitions()))
print("Elements across partitions is:" + str(df.rdd.mapPartitionsWithIndex(lambda ind, x: count_elements(ind, x)).take(3)))

The Code above kept failing with following error

  n = sum(1 for _ in iterator)
  File "/home/dev/wk/pyenv/py3/lib/python3.5/site-packages/pyspark/python/lib/pyspark.zip/pyspark/sql/functions.py", line 40, in _
    jc = getattr(sc._jvm.functions, name)(col._jc if isinstance(col, Column) else col)
AttributeError: 'NoneType' object has no attribute '_jvm'

after removing the import below

from pyspark.sql.functions import *

Code works fine

skewed_large_df has partitions.3
The distribution of elements across partitions is:[(0, 1), (1, 2), (2, 2)]

What is it causing this error and how can I fix it?

pault
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user400058
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    Don't do `import *` as it can mess up your namespace. Do `import pyspark.sql.functions as f` and call the functions from that module using `f.function_name()`. I'm pretty sure you meant to call the builitin `sum()` and not `pyspark.sql.functions.sum()`. That's probably what's causing your issue. – pault Mar 26 '18 at 14:16
  • Thanks for pointing that @pault. very helpful. I would except buildins.py sum method to have precedence over pyspark.sql.functions.sum() method! – user400058 Mar 26 '18 at 17:33
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    I think @pault comment should be posted as answer. – Zbyszek Apr 16 '19 at 12:44
  • Possible duplicate of [pyspark Column is not iterable](https://stackoverflow.com/questions/36924873/pyspark-column-is-not-iterable). – pault Apr 10 '20 at 12:52

1 Answers1

16

This is a great example of why you shouldn't use import *.

The line

from pyspark.sql.functions import *

will bring in all the functions in the pyspark.sql.functions module into your namespace, include some that will shadow your builtins.

The specific issue is in the count_elements function on the line:

n = sum(1 for _ in iterator)
#   ^^^ - this is now pyspark.sql.functions.sum

You intended to call __builtin__.sum, but the import * shadowed the builtin.

Instead, do one of the following:

import pyspark.sql.functions as f

Or 

from pyspark.sql.functions import sum as sum_
pault
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    This is a legit answer. I'm not closing this question. Can you remove your vote close please ? :) – eliasah Apr 17 '19 at 07:52