Migration is a critical issue for python 3 adoption. I think python 3 should have done something similar to Protocol Buffers v3 (proto3) by introducing syntax = "python3" statement and let runtime support both v2 and v3. This allows people migrate one file at a time. In any large scale production environment, this is pretty much the only practical way for migration.

Another critical mistake python 3 made was unicode support. For people who work deeply in i18n support, UTF-8 encoding is the only practical solution. Python 3 string should use UTF-8 much like Go. Just for reference, Google stores almost all data as protobuf, which only support UTF-8. That proves the story.