Had a peek at the 2.7 branch in the web (https://hg.python.org/cpython/file/4234b0dd2a54/Lib/test) and all the tests appear to be testing the behaviour *with* the future directive, not bothering to test the old behaviour. It makes sense - that way, when the future directive becomes permanent, there's no suddenly-failing test - can someone confirm that that's the intention?

The current test simply triggers a StopIteration and verifies that RuntimeError comes up off it, without testing the current behaviour, nor testing any of the aspects that haven't changed. I'm basically assuming that generators themselves are adequately tested elsewhere, such that a bug in the PEP 479 code that breaks generators in any other way should be caught by a test failure someplace else. Can anyone think of other aspects of PEP 479 that need to be tested?