Library compatibility. Many libraries are still not compatible with 1.9. It's good to have interim improvements for 1.8 until the ecosystem has caught up with 1.9, because 1.8 is what actually powers production systems today.

"Many libraries are still not compatible with 1.9"

It's not likely they will remain so for very long

I hope you're right. But I'd rather that the move to 1.9 be based on its merits as enhancements to the language rather than on the lack (in 1.8) of the sorts of optimizations that were made in the article.

I would gladly accept a major library breakage for proper multi-processor support in Python. A better implementation is a significant enhancement.