title: How to detect expired access tokens when creating the session?
body: <p>I have the following test program:</p>
  
  <pre><code>from   rauth.service import OAuth1Service, OAuth2Service
  
  SUPPORTED_SERVICES = {
      'twitter'  : ( 'OAuth1', 'twitter',  'https://api.twitter.com/oauth',        'request_token', 'access_token', 'authorize', 'https://api.twitter.com/1/',  None),
      'facebook' : ( 'OAuth2', 'facebook', 'https://graph.facebook.com/oauth',     None,            'access_token', 'authorize', 'https://graph.facebook.com/', 'https://www.facebook.com/connect/login_success.html'),
      'google'   : ( 'OAuth2', 'google',   'https://accounts.google.com/o/oauth2', None,            'token',        'auth',      None,                          'http://localhost'),
  }
  
  CLIENT_DATA = {
      'twitter'  : ('dummy_client_id', 'dummy_client_secret'),
      'facebook' : ('dummy_client_id', 'dummy_client_secret'),
      'google'   : ('dummy_client_id', 'dummy_client_secret'),
  }
  
  USER_TOKENS = {
      'user1' : {
          'twitter'  : ('dummy_access_token', 'dummy_access_token_secret'),
          'facebook' : ('dummy_access_token', None),
          'google'   : ('dummy_access_token', None),
      }
  }
  
  def test_google(user_id):
      service_id = 'google'
      oauthver, name, oauth_base_url, request_token_url, access_token_url, authorize_url, base_url, redirect_uri = SUPPORTED_SERVICES[service_id]
      request_token_url = oauth_base_url + '/' + (request_token_url or '')
      access_token_url  = oauth_base_url + '/' + access_token_url
      authorize_url     = oauth_base_url + '/' + authorize_url
      client_id, client_secret = CLIENT_DATA[service_id]
      google = OAuth2Service(
          client_id=client_id,
          client_secret=client_secret,
          name=name,
          authorize_url=authorize_url,
          access_token_url=access_token_url,
          base_url=base_url)
      access_token, access_token_secret = USER_TOKENS[user_id][service_id] # access_token_secret only needed for twitter (OAuth1)
      session = google.get_session(access_token)
      user = session.get('https://www.googleapis.com/oauth2/v1/userinfo').json()
      print user
  
  test_google('user1')
  </code></pre>
  
  <p>I have authorized my application to access the google account of <code>user1</code>, and obtained an access_token. That access token has already expired, and the output of my program is:</p>
  
  <pre><code>{u'error': {u'code': 401, u'message': u'Invalid Credentials', u'errors': [{u'locationType': u'header', u'domain': u'global', u'message': u'Invalid Credentials', u'reason': u'authError', u'location': u'Authorization'}]}}
  </code></pre>
  
  <p>I would like to check whether the access token has expired when creating the session, <em>not when requesting data</em>. Is this possible? <strong>How can I verify if a session object is really authorized?</strong></p>
  
  <p>For clarification, what I am trying to do is the following:</p>
  
  <ol>
  <li>First, let the user authorize my application</li>
  <li>Save the access token for future use (in a database, but in the test code this is hardcoded in the script)</li>
  <li>Whenever a later access detects that the token has expired, go back to step 1</li>
  </ol>
  
  <p>I am currently having trouble with step 3. I can of course detect the 401 in the json reply to my GET, but it looks rather cumbersome to be forced to verify all GET accesses. What I am trying to do is to verify that the session is really active when I create it, and then assume for the whole duration of the session object, that it will stay active. Usually this will be just several milliseconds, while my webapp is processing the request and accessing the google API using the OAuth session object.</p>