WeChat access token

def access_token(self):
        """ WeChat access token """
        access_token = self.session.get(self.access_token_key)
        if access_token:
            if not self.expires_at:
                # user provided access_token, just return it
                return access_token

            timestamp = time.time()
            if self.expires_at - timestamp > 60:
                return access_token

        self.fetch_access_token()
        return self.session.get(self.access_token_key)