I have a working updated shutil module, tests pass and the documentation is improved.  I will make sure to make different commits for improving the tests, cleaning up some things, adding tarfile.compression_formats and removing duplication in shutil (see dep bugs), to be sure not to introduce regressions.

Before I finish and post the patch, I’d like feedback on a choice I made.  I don’t think it’s possible to have shutil automatically support all the formats that tarfile does, because of the spelling issue I mentioned.  Here’s what the code would look like (let me know if I should post it elsewhere or as a file to let you get syntax coloring):
          
  _ARCHIVE_FORMATS = {}
  _UNPACK_FORMATS = {}

  for fmt in tarfile.compression_formats:
      code = fmt + 'tar'
      ext = '.' + fmt
      desc = "%s'ed tar file" % fmt
      _ARCHIVE_FORMATS[code] = (_make_tarball, [('compress', fmt)], desc)
      _UNPACK_FORMATS[code] = ([ext], _unpack_tarfile, [], desc)

  # kludge to add alternate extension
  if 'gztar' in _ARCHIVE_FORMATS:
      _UNPACK_FORMATS['gztar'][0].append('.tgz')
      # XXX desc should say "gzip'ed tar file", not "gz'ed"

  # rectify naming incompatibility
  if 'bz2tar' in _ARCHIVE_FORMATS:
      # XXX alternative: make 'bztar' alias for 'bz2tar', but would complicate
      # manipulating the registries
      del _ARCHIVE_FORMATS['bz2tar']
      del _UNPACK_FORMATS['bz2tar']
      desc = "bzip2'ed tar file"
      _ARCHIVE_FORMATS['bztar'] = (_make_tarball, [('compress', 'bz2')], desc)
      _UNPACK_FORMATS['bztar'] = (['.bz2'], _unpack_tarfile, [], desc)

  # now add uncompressed tar and zip file

I really don’t like that code.  Given that tarfile is not extensible at run time, it is not a big deal to have to update shutil whenever we add a compression format to tarfile.  Therefore, I backtracked on my “automatic support” idea but kept a lot of cleanup in did in the code.  Here’s what the code looks like:

  _ARCHIVE_FORMATS = {}
  _UNPACK_FORMATS = {}

  if 'xz' in tarfile.compression_formats:
      desc = "xz'ed tar file"
      # XXX '.xz' is not great, '.tar.xz' would be better
      _ARCHIVE_FORMATS['xztar'] = (_make_tarball, [('compress', 'xz')], desc)
      _UNPACK_FORMATS['xztar'] = (['.xz'], _unpack_tarfile, [], desc)

  if 'bz2' in tarfile.compression_formats:
      desc = "bzip2'ed tar file"
      _ARCHIVE_FORMATS['bztar'] = (_make_tarball, [('compress', 'bz2')], desc)
      _UNPACK_FORMATS['bztar'] = (['.bz2'], _unpack_tarfile, [], desc)

  if 'gz' in tarfile.compression_formats:
      desc = "gzip'ed tar file"
      _ARCHIVE_FORMATS['gztar'] = (_make_tarball, [('compress', 'gz')], desc)
      _UNPACK_FORMATS['gztar'] = (['.gz', '.tgz'], _unpack_tarfile, [], desc)

So, do you agree that “not automated but not ugly” is better than “automated with ugly klutches”?