def _get_xml_et_value(xml_et, key, rettype=str):
    """
    :param xml_et: Elementtree's element
    :param key:
    :param rettype: class, force to convert it from str
    :return: rettype's value
    Note that if there is no keys in xml object, return None
    """
    elm = xml_et.find(key)
    if elm is None:
        return elm

    if isinstance(rettype, str):
        return elm.text
    else:
        return rettype(elm.text)