IdleHistory.py is identical in 2.7 and 3.3. It is only imported in PyShell.py (within class PyShell, line 852 in 3.3). History is only initialized once, with the default output_sep. See #18732 for deleting the unneeded parameter and associated cruft.

The pushed patch adds docstrings and a couple of comments and removes most of the redundant prefixes. It does not change any logic.

The attached patch started as JK's patch with names changed and Ezio's changes. All test methods pass. There are three chunks of History.fetch that are not covered. Two require cyclic=False. The other requires special situations with text or cursor changes after one call to fetch but before the next. I will add these soon and push.