I just heard about PEP479, and I want to prepare my open-source projects for it. 

I have no problem changing the code so it won't depend on StopIteration to stop generators, but I'd also like to test it in my test suite. In Python 3.5 I could use `from __future__ import generator_stop` so the test would be real (i.e. would fail wherever I rely on StopIteration to stop a generator). But I can't really put this snippet in my code because then it would fail on all Python versions below 3.5. 

This makes me think of two ideas:

1. Maybe we should allow `from __future__ import whatever` in code, even if `whatever` wasn't invented yet, and simply make it a no-op? This wouldn't help now but it could prevent these problems in the future. 

2. Maybe introduce a way to do `from __future__ import generator_stop` without including it in code? Maybe a flag to the `python` command? (If something like this exists please let me know.) 

Thanks,

Ram.