Github user zentol commented on the pull request:
https://github.com/apache/flink/pull/202#issuecomment-76979731
yes they are implemented as they are for performance reasons. the python
cogroup grouping logic is actually a direct port of the
SortMergeCoGroupIterator.
it also makes things a bit simpler since you can work on the assumption
that an operators function is not called more than once. if, on the java side,
hasNext() returns false we know that we processed all input data, something you
usually can only say when close() was called.
the coGroupPython* stuiff is generic, will try to come up with a more
suitable name.
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