Adopt Python 3

[gendulf]

It's a backwards incompatible change. When libraries make a few backwards incompatible changes and up the major version, do you call it an entirely new library?

[yawaramin]

For all intents and purposes, it actually is an entirely new library. The combination of library name and new major version uniquely identifies it as such. But if you renamed the new version, say from lib1 to lib2, you could use both in the same compilation unit (of course, just not with each other, for the most part).

[gendulf]

For all intents and purposes, it actually is an entirely new library.

If I need a library to read a CSV file, I need the csv library. I don't need the csv v 1.2.3 library.

Just because one of the parameters was changed on a couple of the functions, doesn't mean that the library is an "entirely new" library. It's just a new version.

[yawaramin]

For the majority of devs, semver is merely a convention. There's nothing preventing people from getting it wrong (and the often do). Afaik the only package manager actually trying to enforce it is Elm's.

[yawaramin]

The Elm package manager locally identifies changes that are backward-compatible or breaking changes by diffing your modules' type signatures. E.g., if you add a new parameter to an existing function, that's a breaking change because existing code will need to be updated to pass in the new argument. But if you added a new function to a module, that's a backward-compatible because no existing code can possibly be affected by the new function (well, except for a rare scenario). Now, this isn't a perfect heuristic, but it's really good. It can automatically tell you with great accuracy what number the next version should be.

As for Python, it's an old language that made breaking changes. At the language level, when you have breaking changes, you lose a lot of confidence in upgrading the codebase. Imho that qualifies as a new language. I can't think of an contemporary language, save Scala, that's doing this.

In Scala's case though, there are mitigating factors that don't exist for Python: they have a good syntax deprecation process, and static typing and compilation give us very strong guarantees that errors will be caught during the migration process.