tarfile.open() allows for specifying both a compression type and a compression level. However, the compresslevel parameter doesn't work when using the xz modes. import tarfile good_archive = tarfile.open('/tmp/dummy.tar.gz', 'w:gz', compresslevel=9) bad_archive = tarfile.open('/tmp/dummy.tar.gz', 'w:xz', compresslevel=9) This gives an error in the tarfile.TarFile constructor: TypeError: __init__() got an unexpected keyword argument 'compresslevel' This appears to be due to tarfile.xzopen() specifying a preset keyword instead of a compresslevel keyword. This matches the LZMAFile compressor, but it means that it doesn't get passed in and is then carried along in the kwargs - see [1]. Changing to lzma.LZMAFile(fileobj or name, mode, preset=compresslevel) seems to fix the issue. Assuming that's the right fix, I'll submit a PR. [1] https://github.com/python/cpython/blob/374c6e178a7599aae46c857b17c6c8bc19dfe4c2/Lib/tarfile.py#L1684-L1699