This is actually expected behaviour of the “argparse”, as well as general Unix CLI programs. See the documentation <https://docs.python.org/3.6/library/argparse.html#arguments-containing>. The general workaround is to use a double-dash separator:

>>> parser.parse_args(['--', '-_'])
Namespace(first='-_')

Example with the Gnu “rm” command:

$ echo "make a file" >-_
$ rm -_
rm: invalid option -- '_'
Try 'rm ./-_' to remove the file '-_'.
Try 'rm --help' for more information.
[Exit 1]
$ rm -- -_  # Double dash also works

Although I suppose the error message could be improved. Currently it looks like it ignores the argument:

>>> parser.parse_args(['-_'])
usage: [-h] first
: error: the following arguments are required: first
__main__.SystemExit: 2