See PEP for full details. Attached is POC patch: behaviour is altered globally (rather than respecting a __future__ directive), and minimal changes are made elsewhere to make the test suite mostly pass (test_generators does not - it'll need more comprehensive edits). Note that PEP 8 is deliberately not followed here; some of the edits ought to involve indenting slabs of code, but instead, "half-level" indentation is used, to keep the patch visibly minimal.