prepend url with base_url unless it's already an absolute URL

def build_url(base_url, path):
    """ prepend url with base_url unless it's already an absolute URL """
    if absolute_http_url_regexp.match(path):
        return path
    elif base_url:
        return "{}/{}".format(base_url.rstrip("/"), path.lstrip("/"))
    else:
        raise ParamsError("base url missed!")