rhettinger: I agree with anthonybaxter; implementing this in a module to import is not possible (unless you want *another* module that works like __future__ imports, i.e. where importing it isn't just a plain import - you could then just as well require to put "this is py3k" as the first line of the file). I wish PEP 244 would have been adopted, then you could "directive python3" at the top.

As for testing: I don't see it as a problem if the test suite only works with the version where the feature is introduced. See test_future* and test_with.

As for PRINT_ITEM: it was always inlined, since day 1. It has grown since, but apparently nobody ever bothered refactoring (on day 1, it was smaller, and the bulk of it was, and still is, in writeobject aka PyFile_WriteObject)