Github user pwendell commented on the pull request:
https://github.com/apache/spark/pull/1526#issuecomment-49809869
I think the reason you find it confusing is because you are interpreting it
an imperative (please preserve X) instead of descriptive (this preserves X). I
always interpreted it as descriptive. I.e. you are doing something that
logically preserves the partitioning of the RDD. E.g. you are answering the
question "Does this map function preserve the partitioning of the underlying
data?".
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