In https://bugs.python.org/issue11354

'argparse: nargs could accept range of options count'

I explored the option allowing regex style range arguments, such as nargs='{2,4}' or an equivalent tuple nargs=(2,4).

But that builds on https://bugs.python.org/issue9849, which seeks better error checking of the `nargs` parameter.  In the current implementation, there is no checking or parsing of the value. The full range of allowable values is determined by its use in 2 methods:

_get_nargs_pattern
_format_args