google python api客户端刷新令牌为空

我试图在google python api客户端中使用刷新令牌,以避免每次都需要用户对应用进行身份验证。

我有以下代码,通过让用户每次进行身份验证来构建健身服务

from apiclient.discovery import build
from oauth2client import tools
from oauth2client.file import Storage
from oauth2client.client import AccessTokenRefreshError
from oauth2client.client import OAuth2WebServerFlow
import httplib2

scope = "https://www.googleapis.com/auth/fitness.activity.read"
flow = OAuth2WebServerFlow(client_id, client_secret, scope, redirect_url)
auth_uri = flow.step1_get_authorize_url()
print auth_uri

token = str(input("Enter token"))

cred = flow.step2_exchange(token)
storage = Storage("credentials_file")
storage.put(cred)

storage = Storage("credentials_file")
cred2 = storage.get()

http = httplib2.Http()
http = cred2.authorize(http)
service = build('fitness', 'v1',http=http)

然而,重复身份验证显然并不理想,所以我试图将其转换为可以使用刷新令牌刷新访问令牌的代码。然而,在运行了这段代码之后,cred。刷新你的代币和信用卡。刷新令牌都是空的。如何获取刷新令牌?