Write help message to file.

        Only called by the __driver_helper() method.

        Looks for the help message in the following order:

            1.  The helper method registered with this command via the @helper
                decorator.
            2.  The doc string of the registered method.
            3.  A default help message basically saying 'no help found'.

        Arguments:
            toks: The list of command followed by its arguments.

        Returns:
            The help message.

        Raises:
             As this function is called via the readline complete callback, any
             errors and exceptions are silently ignored.

def __get_help_message(self, toks):
        """Write help message to file.

        Only called by the __driver_helper() method.

        Looks for the help message in the following order:

            1.  The helper method registered with this command via the @helper
                decorator.
            2.  The doc string of the registered method.
            3.  A default help message basically saying 'no help found'.

        Arguments:
            toks: The list of command followed by its arguments.

        Returns:
            The help message.

        Raises:
             As this function is called via the readline complete callback, any
             errors and exceptions are silently ignored.
        """
        cmd = toks[0]
        if cmd in self._helper_map.keys():
            helper_name = self._helper_map[cmd]
            helper_method = getattr(self, helper_name)
            args = toks[1:] if len(toks) > 1 else []
            return helper_method(cmd, args)

        if cmd in self._cmd_map_all.keys():
            name = self._cmd_map_all[cmd]
            method = getattr(self, name)
            if method.__doc__:
                return textwrap.dedent(method.__doc__)

        return textwrap.dedent('''\
                       No help message is found for:
                       {}
                       '''.format(textwrap.indent(
                           subprocess.list2cmdline(toks), '    ')))