How to use the scrapelib._user_agent function in scrapelib

try:
        from IPython import embed
    except ImportError:
        print('scrapeshell requires ipython >= 0.11')
        return
    try:
        import lxml.html
        USE_LXML = True
    except ImportError:
        USE_LXML = False

    parser = argparse.ArgumentParser(prog='scrapeshell',
                                     description='interactive python shell for'
                                     ' scraping')
    parser.add_argument('url', help="url to scrape")
    parser.add_argument('--ua', dest='user_agent', default=_user_agent,
                        help='user agent to make requests with')
    parser.add_argument('-p', '--postdata', dest='postdata',
                        default=None,
                        help="POST data (will make a POST instead of GET)")
    args = parser.parse_args(orig_argv)

    scraper = Scraper()
    scraper.user_agent = args.user_agent
    url = args.url
    if args.postdata:
        html = scraper.urlopen(args.url, 'POST', args.postdata)
    else:
        html = scraper.urlopen(args.url)

    if USE_LXML:
        doc = lxml.html.fromstring(html.bytes)  # noqa