def extract_favicon(bs4):
    """Extracting favicon url from BeautifulSoup object

    :param bs4: `BeautifulSoup`
    :return: `list` List of favicon urls
    """
    favicon = []
    selectors = ['link[rel="icon"]', 'link[rel="Icon"]', 'link[rel="ICON"]',
        'link[rel^="shortcut"]', 'link[rel^="Shortcut"]',
        'link[rel^="SHORTCUT"]']
    for selector in selectors:
        icons = bs4.select(selector)
        if icons:
            for icon in icons:
                if icon.has_attr('href'):
                    favicon.append(icon['href'])
    return favicon
