I'm at the PyConUS sprints, would appreciate a high-level review / if the approach I took here looks like what you expect. In particular I tried to keep behavior around all the unexpected potential return values frombuffer.write but also addDeprecationWarning/RuntimeWarning for many. I'm planning to work on expanding tests for this tomorrow (clone of BufferedIO blocking test, tests for underlying writers that return all the bad values, partial write tests, etc).

cc:@vstinner,@serhiy-storchaka (commented on the underlying issue, but also was quite a while ago. If there's better experts or someone more appropriate let me know)