Briefly, some of my Python 3000 thoughts. I see quite a bit of enthusiasm in some other blogs. I'm not very enthusiastic. While I understand the need to be able to break backwards compatibility, I am worried about Python forking into two parallel versions (2.x and 3.x) for many, many years. Such a fork can't be good for the Python community.
Why do I worry about Python forking? Because porting code to Python 3 will be hard. The porting instructions discuss the need for a good unit test suite before getting started. This is very wise. In practice of course, while I write unit tests for code all the time, lots of the code I work with doesn't have a great test coverage. There's lots and lots and lots of valuable Python code like that out there. What is going to happen to it? How is it going to be ported?
Even with test coverage, porting over code is going to be a massive amount of work for larger code bases. Considering that Zope 3, a clean, modern, extremely well-tested code base (one of the best test coverages in the Python world) is still on Python 2.4 as it takes time and effort to port to Python 2.5, what is going to happen with Python 3? What does this say about codebases with far less tests?
The result in many cases is that people will put off porting code, as it's too costly. It won't be easy to motivate a customer to pay for porting activity that will bring no new features to them whatsoever. People will therefore continue to run this code on Python 2.x. Since Python 2.x code doesn't work on Python 3.x, it won't be accessible to people who made the jump. Since Python 3.x code doesn't work on Python 2.x, it won't be accessible to those with existing code bases who can't make the jump any time soon. As a result, two Python communities for a period of what I expect to be 5 to 10 years.
I realize there are no answers to these worries beyond those that have already been given. Breaking backwards compatibility may be necessary. It just doesn't make me have a "birthday feeling", as I saw described in another blog. Python 3 is a serious risk to the Python community. Not by far as much as, say, Perl 6, is to the Perl community, but a significant risk nonetheless.
(I also am worried that Abstract Base Classes are throwing away some key benefits of zope.interface, but that is not really a blocker - at least zope.interface can be made to work on Python 3).