The attached patch reimplements gzip's already existing command-line interface using argparse, both to provide command-line help and to avoid manual argument parsing.

Thanks for the patch, but we need to add tests for the current CLI to make sure that we don't break anything when we apply gzip-argparse-cli.patch. See Lib/test/test_tarfile.py and Lib/test/test_calendar.py for example CLI tests.

Hi Berker, Here is my PR for this issue. From this PR, I will add the new CLI with argparse. And after this issue, I will work on https://bugs.python.org/issue34913

Thanks @matrixise for the PR. This seems to be a good first step for both testing the command line interface and also argparse can generate help string which can be used for issue34913.

 New changeset 84eec1199583bcb034e43337bcb8e2b876ebd269 by Julien Palard (Stéphane Wirtel) in branch 'master': bpo-23596: Add unit tests for the command line for the gzip module (GH-9775) https://github.com/python/cpython/commit/84eec1199583bcb034e43337bcb8e2b876ebd269 

And now, I am going to work on the refactoring with argparse

 New changeset 482dc9445d834f1d03c501b45fafd422b6eb8c9c by Miss Islington (bot) in branch '3.6': bpo-23596: Add unit tests for the command line for the gzip module (GH-9775) https://github.com/python/cpython/commit/482dc9445d834f1d03c501b45fafd422b6eb8c9c 

 New changeset 8e23ba021f45df67edd2926a3e3daf6a2a1f0ed8 by Miss Islington (bot) in branch '3.7': bpo-23596: Add unit tests for the command line for the gzip module (GH-9775) https://github.com/python/cpython/commit/8e23ba021f45df67edd2926a3e3daf6a2a1f0ed8 

 New changeset e8bbc52debfd1b28517946d65db257e6b6d92e29 by Julien Palard (Stéphane Wirtel) in branch 'master': bpo-23596: Use argparse for the command line of gzip (GH-9781) https://github.com/python/cpython/commit/e8bbc52debfd1b28517946d65db257e6b6d92e29 

I have added some tests for 3.6, 3.7 and 3.8. But for the new CLI, just for 3.8 now, I am going to close this issue.