Yeah, the % formatting is an artifact of argparse being around before
str.format was available. If argparse became part of the standard
library, it might be reasonable to change to str.format formatting
instead as part of the move.

It would mean that folks would have to modify their code to switch from
the current argparse module to the stdlib one, but maybe that's an
acceptable tradeoff. I'm certainly happy to change things to str.format
if that's agreed on.

Thanks again for looking at this.