For optional flags like --foo-bar, argparse does munge the "dest" to "foo_bar", following optparse. For positional arguments, arpgarse doesn't munge things this way, but if you want the argument named "foo-bar" in help messages and "foo_bar" on the Namespace object, you just need to do something like:

add_argument('foo_bar', metavar='foo-bar', ...)

Perhaps the docs could make this clearer.