def get_patent_id(soup):
    """ Get patent-ID from USPTO patent HTML.

    Parameters
    ----------
    soup : bs4.BeautifulSoup
        Patent HTML.

    Returns
    -------
    patent_id : str
        Patent-ID
    """
    title_tag = soup.findChild('head').find('title')
    _, patent_id = title_tag.text.split(': ')
    return patent_id