(FWIW, "semantics" would be "what it means", so I figure that's not what you wanted to say: from your example, I guess you are saying it's more that it's a different wording — syntax? — for the same meaning)
But it's about setting expectations.
The only problem I ever had with py2 to py3 migration was that it was even possible to have the same codebase run against both, when languages are incompatible to such a degree (most notably, basic type has changed). It basically forced people to make the worst use of the Python dynamic nature (as soon as the stdlib started doing that, there was no going back).
