mode:
'r' or 'r:*' open for reading with transparent compression
'r:'         open for reading exclusively uncompressed
'r:gz'       open for reading with gzip compression
'r:bz2'      open for reading with bzip2 compression
'r:xz'       open for reading with lzma compression
'a' or 'a:'  open for appending, creating the file if necessary
'w' or 'w:'  open for writing without compression
'w:gz'       open for writing with gzip compression
'w:bz2'      open for writing with bzip2 compression
'w:xz'       open for writing with lzma compression

'x' or 'x:'  create a tarfile exclusively without compression, raise
             an exception if the file is already created
'x:gz'       create a gzip compressed tarfile, raise an exception
             if the file is already created
'x:bz2'      create a bzip2 compressed tarfile, raise an exception
             if the file is already created
'x:xz'       create an lzma compressed tarfile, raise an exception
             if the file is already created

'r|*'        open a stream of tar blocks with transparent compression
'r|'         open an uncompressed stream of tar blocks for reading
'r|gz'       open a gzip compressed stream of tar blocks
'r|bz2'      open a bzip2 compressed stream of tar blocks
'r|xz'       open an lzma compressed stream of tar blocks
'w|'         open an uncompressed stream for writing
'w|gz'       open a gzip compressed stream for writing
'w|bz2'      open a bzip2 compressed stream for writing
'w|xz'       open an lzma compressed stream for writing
znothing to open)