def _register_handler(self, subparser, handler):
subparser.set_defaults(_func=handler)<EOL>
Add a handler as a default ``_func`` attribute to a subparser.

        Args
        ----
        subparser : argparse.ArgumentParser
            The subparser to add the handler to.
        handler : function
            The function to add to the subparser, which will be called with the
            namespace returned by the subparser as kwargs.

        Returns
        -------
        None