The crucial lesson from the Python 2 -> 3 transition (which this article is also mainly about) is that incrementality is super valuable. In retrospect, a single

The crucial lesson from the Python 2 -> 3 transition (which this article is also mainly about) is that incrementality is super valuable. In retrospect, a single codebase supporting 2 and 3 is the obvious best option, but that wasn't at all clear in the beginning. (For example, the u"" string syntax, which is crucial for compatibility, wasn't even legal until 3.3!) Moving to Python 3 incrementally is painful, but it's possible. Moving to a totally different programming language incrementally isn't possible, other than when you're starting brand new projects.