Github user mateiz commented on the pull request:
https://github.com/apache/spark/pull/876#issuecomment-44497883
This functionality doesn't fit the definition of a Partitioner as used in
Spark (which requires it to consistently return the same partition for each
key), so it would be confusing to expose it as such. The `repartition` and
`coalesce` methods are the right way to do it.
In particular, Partitioners are also used to decide whether you can
optimize joins and lookups based on a key's partition. This would break that
behavior.
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