Because it's not just syntactic changes, it's implied-semantic changes too. You can't mechanically transform a project and know that it'll work. And you can't

Because it's not just syntactic changes, it's implied-semantic changes too. You can't mechanically transform a project and know that it'll work. And you can't do it gradually, so it's all-or-nothing. (yes, "six" exists, but you still execute one way or another) And you'll have to change the versions of all your libraries, which is not usually a smooth experience in the Python ecosystem. (this is another place where it's "all or nothing", since six can't help you if your dependencies don't all use it + use it correctly) --- It's a huge risk with huge cost for already-working, running code. For new stuff, sure, write it in 3, but 2.7 works fine and has the added benefit of being very well understood by this point.