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

In working on a patch I convinced myself that a better change might just be to document that you can use the *preset* keyword for LZMA compression instead of *compresslevel*.

 New changeset eb2d4a66ff07aa6e51cfaaa31afed31addf76936 by Zackery Spytz in branch 'main': bpo-32695: Docs and tests for compresslevel and preset kwargs in tarfile (GH-21470) https://github.com/python/cpython/commit/eb2d4a66ff07aa6e51cfaaa31afed31addf76936 

 New changeset ede221e51796f3f5471278aabd9105870f79dfb5 by Miss Islington (bot) in branch '3.9': bpo-32695: Docs and tests for compresslevel and preset kwargs in tarfile (GH-21470) (GH-27674) https://github.com/python/cpython/commit/ede221e51796f3f5471278aabd9105870f79dfb5 

 New changeset d5c8ad24716d146ffa025e09dad85e5a1bac5c77 by Miss Islington (bot) in branch '3.10': bpo-32695: Docs and tests for compresslevel and preset kwargs in tarfile (GH-21470) https://github.com/python/cpython/commit/d5c8ad24716d146ffa025e09dad85e5a1bac5c77