I found an issue that needs to be addressed: if there is an error while the TarFile object is opened for writing, we cannot simply call TarFile.close() in the __exit__() method. close() would try to finalize the archive, i.e. write two zero end-of-archive blocks and a number of padding blocks.

I changed __exit__() to call close() only if everything went fine. If there was an exception only the most basic cleanup is done.

I added more tests and adapted the docs.