I saw some people mentioning changes like changing print statement to print function. That was actually one of the most trivial changes and you could import pri

I saw some people mentioning changes like changing print statement to print function. That was actually one of the most trivial changes and you could import print_function from __future__ which worked like pragmas. Similar problem could be with changing behavior for divisions (which actually was more challenging) but similarly you could enable that behavior. The main problem with migration though was addition of Unicode. You can't just enable it on file by file basis, because once you enable the new behavior in a single file you, will start passing arguments in Unicode to other code in other files and if that code wasn't adapted or will break. And it was even worse than that because that problem extended to your dependencies as well. Ideally dependencies should be updated first, then your application, but since python 2 was still supported (for a decade after python 3 was released) then there was no motivation to do it. And if that wasn't enough python 2 already had Unicode support added, but that implementation was incorrect, so even if you imported Unicode_literals from __future__ you potentially broke compatibility with existing python 2 dependencies without guarantee that your code will work on python 3. IMO that particular change couldn't be done with pragmas, the core issue is that python 3 put a clear separation between text and binary data, but Python 2 mangled them together. That still was true even when you used Unicode in python 2. The proper way to perform the migration IMO would be to type annotate the code. And then run mypy check in python 3 mode.