How to use the jupyterlab.commands.pjoin function in jupyterlab

uri = url_escape(ujoin('/lab/tree', *relpath.split(os.sep)))
        nbapp.default_url = uri
        nbapp.file_to_run = ''

    if core_mode:
        app_dir = HERE
        logger.info(CORE_NOTE.strip())
        ensure_core(logger)

    elif dev_mode:
        app_dir = DEV_DIR
        ensure_dev(logger)
        if not watch_mode:
            logger.info(DEV_NOTE)

    config.app_settings_dir = pjoin(app_dir, 'settings')
    config.schemas_dir = pjoin(app_dir, 'schemas')
    config.themes_dir = pjoin(app_dir, 'themes')
    config.workspaces_dir = workspaces_dir
    info = get_app_info(app_dir)
    config.app_version = info['version']
    public_url = info['publicUrl']
    if public_url:
        config.public_url = public_url
    else:
        config.static_dir = pjoin(app_dir, 'static')

    config.user_settings_dir = user_settings_dir

    # The templates end up in the built static directory.
    config.templates_dir = pjoin(app_dir, 'static')

nbapp.default_url = uri
        nbapp.file_to_run = ''

    if core_mode:
        app_dir = HERE
        logger.info(CORE_NOTE.strip())
        ensure_core(logger)

    elif dev_mode:
        app_dir = DEV_DIR
        ensure_dev(logger)
        if not watch_mode:
            logger.info(DEV_NOTE)

    config.app_settings_dir = pjoin(app_dir, 'settings')
    config.schemas_dir = pjoin(app_dir, 'schemas')
    config.themes_dir = pjoin(app_dir, 'themes')
    config.workspaces_dir = workspaces_dir
    info = get_app_info(app_dir)
    config.app_version = info['version']
    public_url = info['publicUrl']
    if public_url:
        config.public_url = public_url
    else:
        config.static_dir = pjoin(app_dir, 'static')

    config.user_settings_dir = user_settings_dir

    # The templates end up in the built static directory.
    config.templates_dir = pjoin(app_dir, 'static')

    if watch_mode:

nbapp.file_to_run = ''

    if core_mode:
        app_dir = HERE
        logger.info(CORE_NOTE.strip())
        ensure_core(logger)

    elif dev_mode:
        app_dir = DEV_DIR
        ensure_dev(logger)
        if not watch_mode:
            logger.info(DEV_NOTE)

    config.app_settings_dir = pjoin(app_dir, 'settings')
    config.schemas_dir = pjoin(app_dir, 'schemas')
    config.themes_dir = pjoin(app_dir, 'themes')
    config.workspaces_dir = workspaces_dir
    info = get_app_info(app_dir)
    config.app_version = info['version']
    public_url = info['publicUrl']
    if public_url:
        config.public_url = public_url
    else:
        config.static_dir = pjoin(app_dir, 'static')

    config.user_settings_dir = user_settings_dir

    # The templates end up in the built static directory.
    config.templates_dir = pjoin(app_dir, 'static')

    if watch_mode:
        logger.info('Starting JupyterLab watch mode...')

app_dir = DEV_DIR
        ensure_dev(logger)
        if not watch_mode:
            logger.info(DEV_NOTE)

    config.app_settings_dir = pjoin(app_dir, 'settings')
    config.schemas_dir = pjoin(app_dir, 'schemas')
    config.themes_dir = pjoin(app_dir, 'themes')
    config.workspaces_dir = workspaces_dir
    info = get_app_info(app_dir)
    config.app_version = info['version']
    public_url = info['publicUrl']
    if public_url:
        config.public_url = public_url
    else:
        config.static_dir = pjoin(app_dir, 'static')

    config.user_settings_dir = user_settings_dir

    # The templates end up in the built static directory.
    config.templates_dir = pjoin(app_dir, 'static')

    if watch_mode:
        logger.info('Starting JupyterLab watch mode...')

        # Set the ioloop in case the watch fails.
        nbapp.ioloop = IOLoop.current()
        if dev_mode:
            watch_dev(logger)
        else:
            watch(app_dir, logger)
            page_config['buildAvailable'] = False

config.app_settings_dir = pjoin(app_dir, 'settings')
    config.schemas_dir = pjoin(app_dir, 'schemas')
    config.themes_dir = pjoin(app_dir, 'themes')
    config.workspaces_dir = workspaces_dir
    info = get_app_info(app_dir)
    config.app_version = info['version']
    public_url = info['publicUrl']
    if public_url:
        config.public_url = public_url
    else:
        config.static_dir = pjoin(app_dir, 'static')

    config.user_settings_dir = user_settings_dir

    # The templates end up in the built static directory.
    config.templates_dir = pjoin(app_dir, 'static')

    if watch_mode:
        logger.info('Starting JupyterLab watch mode...')

        # Set the ioloop in case the watch fails.
        nbapp.ioloop = IOLoop.current()
        if dev_mode:
            watch_dev(logger)
        else:
            watch(app_dir, logger)
            page_config['buildAvailable'] = False

        config.cache_files = False

    base_url = web_app.settings['base_url']
    build_url = ujoin(base_url, build_path)