I've found 3 broad classes of code bases when migrating: 1. Trivial or easy. Probably started using best 2/3 compatibility practices easily on, or the nature o

I've found 3 broad classes of code bases when migrating: 1. Trivial or easy. Probably started using best 2/3 compatibility practices easily on, or the nature of the code lends itself to 2to3. 2. Not able to switch without effort, but able to be slowly ported to 2/3 with six and the like. 3. Hopelessly intractable. Either too much bad practices in string handling, or tied to a legacy system. No way to port it without risking subtle bugs. Usually this is due to playing fast and loose with string types, even though we have known better since well before python 3.