[RussianCow]

Kind of, but probably not in the way you're thinking. All I meant is that we transitioned each of our services from Python 2 to code that was compatible with both Python versions, one by one. We tackled one aspect of Python 3 support at a time, e.g. imports, strings, integer division, third-party libraries, etc. Between future imports and utility librariess like six, most things were fairly easy to make compatible across both versions.

Once we did as much as we could do on Python 2, we had one of our engineers maintain a separate Python 3 branch in a staging environment for a while (maybe two weeks?) that we did some heavy QA on. And once we were fairly confident in the results, we merged and released that over a weekend and fixed issues as they came up.

We did all of this once for each service, but the bulk of the work (>80%) was our legacy monolith. I think the whole thing would have taken way less time with better test coverage, but alas. :)