class USTCCASBackend(object):
    def get_sno(self, ticket, service_url):
        """
        Use ticket to get SNO in USTC CAS
        :param ticket:
        :param service_url:
        :return: SNO (upper letters)
        """
        # check ticket valid
        if len(ticket) is not 35:
            return None

        pattern = re.compile(r'^ST-\w{32}$')
        if pattern.match(ticket) is None:
            return None

        # use ticket
        info = None
        api_url = settings.API_URL.format(ticket=ticket, service=service_url)
        try:
            r = requests.get(api_url)
            info = r.text
        except:
            return None

        # format return info
        if info:
            pattern = re.compile(r'<cas:user>([a-zA-Z]{2}\d{8})</cas:user>')
            match = pattern.search(info)
            if match:
                sno = match.group(1).upper()
                return sno
            else:
                return None
        else:
            return None

    def authenticate(self, token=None):

        sno = self.get_sno(token, settings.SERVICE_URL)

        if sno is not None:
            try:
                user = User.objects.get(username=sno)
            except User.DoesNotExist:
                user = User(username=sno, password=sno)
                user.save()
            return user
        return None

    def get_user(self, user_id):
        try:
            return User.objects.get(pk=user_id)
        except User.DoesNotExist:
            return None