I'd like to re-emphasize that it is best to keep the whole thing as simple and straight-forward as possible. Offer some basic operations and that's it.

Although I am pretty accustomed to the original tar command line, I think we should copy zipfile's interface. It makes more sense to offer some kind of unified "Python" command line approach for archive access than keeping to old traditions.

I agree with Victor that we don't really need support for stdin/stdout. It only complicates matters. 

If everybody still votes for stdin/stdout, I'd like to point out that tarfile supports compression detection for streams. It would be best to use mode="r|*" throughout because it works for both normal files and stdin. Use mode="w|(compression)" for writing to files and stdout accordingly.

If we do not support stdin/stdout we no longer need all these compression options because for reading we do autodetection and for writing we could deduce the compression from the file extension (which is just some kind of autodetection too).

Another side note: We should be aware of the effects discussed in issue17102 and issue1044. In my opinion tarfile as a library is obligated to behave like that, but maybe that's not acceptable for a command line tool.