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From Bryan Cutler <cutl...@gmail.com>
Subject [DISCUSS] Remove sorting of fields in PySpark SQL Row construction
Date Mon, 04 Nov 2019 22:28:50 GMT
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

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