It should be easy to write a subclass of Action, or append Action, that does what you want.  It just needs a different `__call__` method.  You just need a way of identifying an default that needs to be overwritten as opposed to appended to.


    def __call__(self, parser, namespace, values, option_string=None):
        current_value = getattr(namspace, self.dest)
        if 'current_value is default':
            setattr(namespace, self.dest, values)
            return
        else:
            # the normal append action
            items = _copy.copy(_ensure_value(namespace, self.dest, []))
            items.append(values)
            setattr(namespace, self.dest, items)

People on StackOverFlow might have other ideas.