Single codebase compatibility meaning that you can have python2 and python3 code in the same application? Isn't that significantly harder with an interpreted language or am I missing something?
It's a mostly solved problem with Racket. What they probably should have done was have python 2 code somehow declare itself as python 2 (Racket does this with #lang at the top of files). Then, just have a python 2 compatibility layer that works in two steps. First step is to compile/parse it into a similar form as python 3. Additionally, provide a small python 2 runtime which provides different versions of the functions which changed from 2 to 3. I think the two steps are important because some stuff is easier to solve via compilation like "print" while other stuff may be only possible at runtime like strings being unicode.
You would still have some differences which can't be papered over, but it would have made writing code that works in both python 2 and 3 much easier.
Single-codebase compatibility means that the same code can run under both Python 2 and Python 3.
The initial expectation was that translation tools would solve this problem, but it didn’t really work out that way. Adding language features and library shims to make it possible to write pidgin Python that would run under either version meant that you could migrate libraries and parts of large codebases one at a time until the whole thing ran under Python 3.
That's the main working solution I found: code works in both versions.
The problem is that it's way trickier than it should be. Has they made that relatively easy, the Python 2->3 transition would have had a much smoother "normal" upgrade process.
