Launchpad still requires Python 2, which in 2020 is a bit of a problem. Unlike a lot of the rest of 2020, though, there’s good reason to be optimistic about progress.

I’ve been porting Python 2 code to Python 3 on and off for a long time, from back when I was on the Ubuntu Foundations team and maintaining things like the Ubiquity installer. When I moved to Launchpad in 2015 it was certainly on my mind that this was a large body of code still stuck on Python 2. One option would have been to just accept that and leave it as it is, maybe doing more backporting work over time as support for Python 2 fades away. I’ve long been of the opinion that this would doom Launchpad to being unmaintainable in the long run, and since I genuinely love working on Launchpad - I find it an incredibly rewarding project - this wasn’t something I was willing to accept. We’re already seeing some of our important dependencies dropping support for Python 2, which is perfectly reasonable on their terms but which is starting to become a genuine obstacle to delivering important features when we need new features from newer versions of those dependencies. It also looks as though it may be difficult for us to run on Ubuntu 20.04 LTS (we’re currently on 16.04, with an upgrade to 18.04 in progress) as long as we still require Python 2, since we have some system dependencies that 20.04 no longer provides. And then there are exciting new features like type hints and async/await that we’d like to be able to use.

However, until last year there were so many blockers that even considering a port was barely conceivable. What changed in 2019 was sorting out a trifecta of core dependencies. We ported our database layer, Storm. We upgraded to modern versions of our Zope Toolkit dependencies (after contributing various fixes upstream, including some substantial changes to Zope’s test runner that we’d carried as local patches for some years). And we ported our Bazaar code hosting infrastructure to Breezy. With all that in place, a port seemed more of a realistic possibility.

Still, even with this, it was never going to be a matter of just following some standard porting advice and calling it good. Launchpad has almost a million lines of Python code in its main git tree, and around 250 dependencies of which a number are quite Launchpad-specific. In a project that size, not only is following standard porting advice an extremely time-consuming task in its own right, but just about every strange corner case is going to show up somewhere. (Did you know that StringIO.StringIO(None) and io.StringIO(None) do different things even after you account for the native string vs. Unicode text difference? How about the behaviour of .union() on a subclass of frozenset?) Launchpad’s test suite is fortunately extremely thorough, but even just starting up the test suite involves importing most of the data model code, so before you can start taking advantage of it you have to make a large fraction of the codebase be at least syntactically-correct Python 3 code and use only modules that exist in Python 3 while still working in Python 2; in a project this size that turns out to be a large effort on its own, and can be quite risky in places.

Canonical’s product engineering teams work on a six-month cycle, but it just isn’t possible to cram this sort of thing into six months unless you do literally nothing else, and “please can we put all feature development on hold while we run to stand still” is a pretty tough sell to even the most understanding management. Fortunately, we’ve been able to grow the Launchpad team in the last year or so, and so it’s been possible to put “Python 3” on our roadmap on the understanding that we aren’t going to get all the way there in one cycle, while still being able to do other substantial feature development work as well.

So, with all that preamble, what have we done this cycle? We’ve taken a two-pronged approach. From one end, we identified 147 classes that needed to be ported away from some compatibility code in our database layer that was substantially less friendly to Python 3: we’ve ported 38 of those, so there’s clearly a fair bit more to do, but we were able to distribute this work out among the team quite effectively. From the other end, it was clear that it would be very inefficient to do general porting work when any attempt to even run the test suite would run straight into the same crashes in the same order, so I set myself a target of getting the test suite to start up, and started hacking on an enormous git branch that I never expected to try to land directly: instead, I felt free to commit just about anything that looked reasonable and moved things forward even if it was very rough, and every so often went back to tidy things up and cherry-pick individual commits into a form that included some kind of explanation and passed existing tests so that I could propose them for review.

This strategy has been dramatically more successful than anything I’ve tried before at this scale. So far this cycle, considering only Launchpad’s main git tree, we’ve landed 137 Python-3-relevant merge proposals for a total of 39552 lines of git diff output, keeping our existing tests passing along the way and deploying incrementally to production. We have about 27000 more lines of patch at varying degrees of quality to tidy up and merge. Our main development branch is only perhaps 10 or 20 more patches away from the test suite being able to start up, at which point we’ll be able to get a buildbot running so that multiple developers can work on this much more easily and see the effect of their work. With the full unlanded patch stack, about 75% of the test suite passes on Python 3! This still leaves a long tail of several thousand tests to figure out and fix, but it’s a much more incrementally-tractable kind of problem than where we started.

Finally: the funniest (to me) bug I’ve encountered in this effort was the one I encountered in the test runner and fixed in zopefoundation/zope.testrunner#106: IDs of failing tests were written to a pipe, so if you have a test suite that’s large enough and broken enough then eventually that pipe would reach its capacity and your test runner would just give up and hang. Pretty annoying when it meant an overnight test run didn’t give useful results, but also eloquent commentary of sorts.