How to use the fbs._state.COMMANDS.items function in fbs

def _get_cmdline_parser():
    # Were we invoked with `python -m fbs`?
    is_python_m_fbs = splitext(basename(sys.argv[0]))[0] == '__main__'
    if is_python_m_fbs:
        prog = '%s -m fbs' % basename(sys.executable)
    else:
        prog = None
    parser = ArgumentParser(prog=prog, description='fbs')
    subparsers = parser.add_subparsers()
    for cmd_name, cmd_fn in COMMANDS.items():
        cmd_parser = subparsers.add_parser(cmd_name, help=cmd_fn.__doc__)
        argspec = getfullargspec(cmd_fn)
        args = argspec.args or []
        defaults = argspec.defaults or ()
        args_without_defaults = args[:1-len(defaults)]
        args_with_defaults = args[-len(defaults):]
        for arg in args_without_defaults:
            cmd_parser.add_argument(arg)
        for arg, default in zip(args_with_defaults, defaults):
            if not isinstance(default, bool):
                raise FbsError(
                    'Error in command %r: Only booleans are currently '
                    'supported as optional arguments.' % cmd_name
                )
            cmd_parser.add_argument(
                '--' + arg, action='store_' + str(not default).lower()