Yep, the question is whether any of the "raise StopIteration" lines are actually non-local flow control. If they're local, then it's easy: mechanical replacement with "return" and it becomes compatible with all versions (unless it has a value attached to it, as "return x" doesn't work in Py2). But if they're non-local, some refactoring will need to be done.

In any case, my line of argument is: A generator function is not an iterator's __next__ method, ergo iterator protocol does not apply. Use of StopIteration is a hack that happens to work because of how generator functions are implemented (a thin wrapper around an iterator), but it's not part of the *concept* of a generator function.