import argparse
import sys

def main(argv=None):

    if argv is None:
        argv = sys.argv[1:]

    parser = argparse.ArgumentParser()

    parser.add_argument('foo',
                      #  nargs='*', when present, works as intended
                        action='extend')

    args  = parser.parse_args(argv)

    print(args)


if __name__ == "__main__":
    main(['test_string']) # prints Namespace(foo=['t', 'e', 's', 't', '_', 's', 't', 'r', 'i', 'n', 'g'])