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From Shane Knapp <skn...@berkeley.edu>
Subject Re: [DISCUSS] Remove sorting of fields in PySpark SQL Row construction
Date Fri, 08 Nov 2019 02:54:26 GMT
+1

On Thu, Nov 7, 2019 at 6:08 PM Hyukjin Kwon <gurwls223@gmail.com> wrote:
>
> +1
>
> 2019년 11월 6일 (수) 오후 11:38, Wenchen Fan <cloud0fan@gmail.com>님이
작성:
>>
>> Sounds reasonable to me. We should make the behavior consistent within Spark.
>>
>> On Tue, Nov 5, 2019 at 6:29 AM Bryan Cutler <cutlerb@gmail.com> wrote:
>>>
>>> Currently, when a PySpark Row is created with keyword arguments, the fields are
sorted alphabetically. This has created a lot of confusion with users because it is not obvious
(although it is stated in the pydocs) that they will be sorted alphabetically. Then later
when applying a schema and the field order does not match, an error will occur. Here is a
list of some of the JIRAs that I have been tracking all related to this issue: SPARK-24915,
SPARK-22232, SPARK-27939, SPARK-27712, and relevant discussion of the issue [1].
>>>
>>> The original reason for sorting fields is because kwargs in python < 3.6 are
not guaranteed to be in the same order that they were entered [2]. Sorting alphabetically
ensures a consistent order. Matters are further complicated with the flag _from_dict_ that
allows the Row fields to to be referenced by name when made by kwargs, but this flag is not
serialized with the Row and leads to inconsistent behavior. For instance:
>>>
>>> >>> spark.createDataFrame([Row(A="1", B="2")], "B string, A string").first()
>>> Row(B='2', A='1')
>>> >>> spark.createDataFrame(spark.sparkContext.parallelize([Row(A="1",
B="2")]), "B string, A string").first()
>>> Row(B='1', A='2')
>>>
>>> I think the best way to fix this is to remove the sorting of fields when constructing
a Row. For users with Python 3.6+, nothing would change because these versions of Python ensure
that the kwargs stays in the ordered entered. For users with Python < 3.6, using kwargs
would check a conf to either raise an error or fallback to a LegacyRow that sorts the fields
as before. With Python < 3.6 being deprecated now, this LegacyRow can also be removed at
the same time. There are also other ways to create Rows that will not be affected. I have
opened a JIRA [3] to capture this, but I am wondering what others think about fixing this
for Spark 3.0?
>>>
>>> [1] https://github.com/apache/spark/pull/20280
>>> [2] https://www.python.org/dev/peps/pep-0468/
>>> [3] https://issues.apache.org/jira/browse/SPARK-29748



-- 
Shane Knapp
UC Berkeley EECS Research / RISELab Staff Technical Lead
https://rise.cs.berkeley.edu

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