Hovmöller: Moving a large and old codebase to Python3

Anders Hovmiller has posted an account of migrating a large application to Python 3. There were multiple steps on the journey and plenty of lessons learned. "Our philosophy was always to go py2 -> py2/py3 -> py3 because we just could not realistically do a big bang in production, an intuition that was proven right in surprising ways. This meant that 2to3 was a non starter which I think is probably common. We tried a while to use 2to3 to detect Python 3 compatibility issues but quickly found that untenable too. Basically it suggests changes that will break your code in Python 2. No good.The conclusion was to use six, which is a library to make it easy to build a codebase that is valid in both in Python 2 and 3."