Using the refresh token to obtain the access token from Google OAuth 2.0 server

c2dm.py

import logging
import urllib
import urllib2
import json

log = logging.getLogger(__name__)

def authenticate(refresh_token, client_id, client_secret):
    """Authenticates with the Google servers to get authentication token
    for C2DM API calls.

    Returns the authentication token, if successful.
    """

    data = urllib.urlencode({
        'grant_type': 'refresh_token',
        'client_id': client_id,
        'client_secret': client_secret,
        'refresh_token': refresh_token,
    })
    req = urllib2.Request('https://accounts.google.com/o/oauth2/token', data)

    try:
        resp = urllib2.urlopen(req)
        return json.loads(resp.read()).get('access_token', None)
    except urllib2.HTTPError, e:
        log.error('HTTP error with status code %d occured: %s',
            e.code, e.read(), exc_info=True)
    except:
        log.error('Failed to authenticate with Google.', exc_info=True)
        return None

okay but how do i supposed to acceess accurate refresh_token , if the way storing it into client ist secure and access_token already retired then how do i supposed to obtain it ?