How to use the parsel.selector.create_root_node function in parsel

def _get_form(response, formname, formid, formnumber, formxpath):
    """Find the form element """
    root = create_root_node(response.text, lxml.html.HTMLParser,
                            base_url=get_base_url(response))
    forms = root.xpath('//form')
    if not forms:
        raise ValueError("No <form> element found in %s" % response)

    if formname is not None:
        f = root.xpath('//form[@name="%s"]' % formname)
        if f:
            return f[0]

    if formid is not None:
        f = root.xpath('//form[@id="%s"]' % formid)
        if f:
            return f[0]

    # Get form element from xpath, if not found, go up</form>