No offense to Guido, and I love Python, but the transition from Python 2 to Python 3 is a reference point for me in terms of how NOT to update a programming language. Noone should ever be attacked, but surely someone deserves some criticism.
For a new programmer, it can be a very discouraging experience having to debug even your first hello world program even though it perfectly matches the code in the tutorial.
The easiest things can be done by automatic translation, like 2to3.
The remaining parts get increasingly difficult, because Python 2 objects have to know how they are interpreted in Python 3, and vice versa.
A simper step would be to write an AST transformer to instrument Python 2 code and identify the non-Python 3-compatible uses of, for example, keys() and values(). That's almost as hard as writing a Python 3 implementation in Python 2, and I couldn't convince myself that it made financial sense given that it would still miss a lot of corner cases.
What you can do is use libraries like six to make code run just fine in both versions, and six-ify on a per file basis. This works pretty well.
AFAICT it allows for optional binding. I suppose it might seem like 'cargo cult' because Rust and Swift have 'if let'?