Note that there is a way to get fully automated support for tar formats: tarfile could expose, in addition to the list compression_formats, another structure with the descriptions (e.g. “gzip’ed tar file”) and file extensions (e.g. ['.gz', '.tgz'] —no, it’s not '.tar.gz', which is unfortunate, and could cause Lars to reject that idea).  I’m just putting this here for reference, but my preference is still for the second idea I talk about in my precedent message.