I work in one of the largest Python shops in the United States. We don't have 100% test coverage (I don't know anyone who does). So there will be stuff breaking. With hundreds of thousands of lines of Python code, the testing/fixing burden would be enormous. It just doesn't make business sense to move. This codebase will never be on Python3. And I think it would be a serious mistake even if it could be done, as the Python3 "migration" isn't done, until it's actually happened. Right now the numbers I've seen are that 10-20% of the Python userbase is on 3.x. The future for Python3 is sketchy at best, those support dates are nothing more than political propaganda. If they meant something, they wouldn't have already pushed it back to 2020. Even if it did matter, code doesn't just stop working when the ball drops in 2020.

So if I could snap my fingers and port us over to 3, and have no breakage (which would cost us millions if not sink the entire business)- I still wouldn't do it. The best strategy is keep on using Python2, which with ~80% of the userbase still at this point in 2015. Python 3.0 was released late 2008. It would be a mistake to switch even if it were risk-free.

We will be using Python2, and bringing Go in-house for new components where we would have used Python3 in an alternate universe where the migration wasn't botched by the CPython core dev team.