> I think we can restructure the code a bit to reduce the work involved in adding a new format, though.
> Maybe something like this: [snip]
Thanks, I’ve applied your obvious refactor in my clone :)  For clarity, I will split my patch into many patches for different reports (bug fixes and doc improvements for 3.2, then code cleanup in 3.3, then adding xz).

> By the way, is there any reason why you've used ".gz" instead of ".tar.gz" as the extension for "gztar"
> in your proposed change? The current version of shutil.py uses ".tar.gz", but has ".bz2" for "bztar".
Yes, I confused the two formats.  I reported the '.bz2' bug (should be '.tar.bz2') and will fix it.

> Also, I notice that _make_tarball() will need to be updated to add the info for xz to the tar_compression
> and compress_ext dictionaries.
I ripped them off.  I could have pasted my full patch to save you time, sorry!

> For the "xztar" format, you should also perhaps recognize the ".txz" extension
Done.  Google finds a number of matches.  I won’t add '.tbz' though, as I don’t know if it’s bzip or bzip2, and as it does not seem used much.