from optparse import OptionParser, Option

class CustomAdd(Option):
    ACTIONS = Option.ACTIONS + ("custom_add",)
    STORE_ACTIONS = Option.STORE_ACTIONS + ("custom_add",)
    TYPED_ACTIONS = Option.TYPED_ACTIONS + ("custom_add",)
    ALWAYS_TYPED_ACTIONS = Option.ALWAYS_TYPED_ACTIONS + ("custom_add",)

    def take_action(self, action, dest, opt, value, values, parser):
        if action == "custom_add":
            values.ensure_value(dest, set()).add(value)
        else:
            Option.take_action(self, action, dest, opt, parser, value, values)

parser = OptionParser(option_class=CustomAdd)
parser.add_option("-a", "--add", dest="test", action="custom_add")

opts, args = parser.parse_args()

print(opts.test)