[masklinn]

> Whereas... imagine if Python 3 rolled out simply as an optional "mode of operation" on top of Python 2.

It's easy to imagine: Python 3 would not have existed. At all.

Python 3 changed fundamental language semantics in ways which went way beyond syntax, that's why the core team shoveled so many changes in: they were breaking the language in a way which didn't brook mere syntactic opt-ins, this meant additional changes were minor issues easily handled compared to the fundamental difficulty of overhauling the language's entire string model and significant parts of the object model.

> If you opt-in on this particular file, the syntax requirements are different.

You're about 20 years late to the party, welcome to Python 2.1: https://docs.python.org/3/whatsnew/2.1.html#pep-236-future-d...