Just to add some perspective from another language: Ruby had a similar, traumatic rift when transitioning from 1.8 to 1.9. I don't think it's a coincidence that -- like Python 2.x to 3.x -- Unicode-handling was one of the changes...so even though Ruby 1.8 to 1.9 had fewer compatibility breaks than Python 2.x to 3.x, it still pissed off a lot of people.

But what made the community jump, IMO, was the fact that the Rails maintainers announced that they would be upgrading to 1.9 [0]. And since there is a very small subset of Ruby users who don't use Rails, that was the end of discussion.

Is there any library in Python that enjoys as much dominance over the language as Rails does to Ruby? Not from what I can tell...And virtually all of the big mindshare libraries in Python have made the transition (e.g. NumPy, Django)...So I agree with OP that making libraries commit to 3.x-or-else is the way to encourage adoption of 3.x...but I just don't see it working as well as it did for Rails/Ruby. That's not necessarily a bad thing, per se, in the sense that it shows the diversity of Python and its use-cases, versus Ruby and its majority use-case of Rails. But forcing the adoption of a version upgrade is one situation in which a mono-culture has the advantage (also, see iOS vs Android).

[0] http://yehudakatz.com/2009/07/17/what-do-we-need-to-get-on-r...