Google API：使用 oauth2client.client 从刷新令牌获取凭据

我正在使用google官方oauth2client.client来访问google 加上API。我有一个存储在数据库中的刷新令牌（不会过期），并且需要 从中重新创建临时“凭据”（访问令牌）。

但我无法找到一种方法来使用谷歌提供的官方库来做到这一点。

所以我绕过了它：使用 urllib 访问 API，它给了我一个新的 来自refresh_token的access_token。使用 access_token 然后我可以使用该库。

我肯定错过了什么！

from apiclient import discovery
from oauth2client.client import AccessTokenCredentials
from urllib import urlencode
from urllib2 import Request , urlopen, HTTPError
import json

# ==========================================

def access_token_from_refresh_token(client_id, client_secret, refresh_token):
  request = Request('https://accounts.google.com/o/oauth2/token',
    data=urlencode({
      'grant_type':    'refresh_token',
      'client_id':     client_id,
      'client_secret': client_secret,
      'refresh_token': refresh_token
    }),
    headers={
      'Content-Type': 'application/x-www-form-urlencoded',
      'Accept': 'application/json'
    }
  )
  response = json.load(urlopen(request))
  return response['access_token']

# ==========================================

access_token = access_token_from_refresh_token(CLIENT_ID, CLIENT_SECRET, REFRESH_TOKEN)

# now I can use the library properly
credentials = AccessTokenCredentials(access_token, "MyAgent/1.0", None)
http = credentials.authorize(httplib2.Http())
service = discovery.build('plus', 'v1', http=http)
google_request = service.people().get(userId='me')
result = google_request.execute(http=http)

I use: oauth2client.client.GoogleCredentials http://oauth2client.readthedocs.io/en/latest/source/oauth2client.client.html#oauth2client.client.GoogleCredentials

    cred = oauth2client.client.GoogleCredentials(access_token,client_id,client_secret,
                                          refresh_token,expires_at,"https://accounts.google.com/o/oauth2/token",some_user_agent)
    http = cred.authorize(httplib2.Http())
    cred.refresh(http)
    self.gmail_service = discovery.build('gmail', 'v1', credentials=cred)