Bridging OAuth 2.0 objects between GData and Discovery

My colleague +Takashi Matsuo and I recently gave a talk about using OAuth2Decorator (from the google-api-python-client library) with request handlers in Google App Engine. Shortly after, a Stack Overflow question sprung up asking about the right way to use the decorator and, as a follow up,  if the decorator could be used with the Google Apps Provisioning API. As I mentioned in my answer,

The Google Apps Provisioning API is a Google Data API...As a result, you'll need to use the gdata-python-client library to use the Provisioning API. Unfortunately, you'll need to manually convert from a oauth2client.client.OAuth2Credentials object to a gdata.gauth.OAuth2Token object to use the same token for either one.

Instead of making everyone and their brother write their own, I thought I'd take a stab at it and write about it here. The general philosophy I took was that the token subclass should be 100% based on an OAuth2Credentials object:

• the token constructor simply takes an OAuth2Credentials object
• the token refresh updates the OAuth2Credentials object set on the token
• values of the current token can be updated directly from the OAuth2Credentials object set on the token

Starting from the top, we'll use two imports:

import httplib2
from gdata.gauth import OAuth2Token

The first is needed to refresh an OAuth2Credentials object using the mechanics native to google-api-python-client, and the second is needed so we may subclass the gdata-python-client native token class.

As I mentioned, the values should be updated directly from an OAuth2Credentials object, so in our constructor, we first initialize the values to None and then call our update method to actual set the values. This allows us to write less code, because, repeating is bad (I think someone told me that once?).

class OAuth2TokenFromCredentials(OAuth2Token):
  def __init__(self, credentials):
    self.credentials = credentials
    super(OAuth2TokenFromCredentials, self).__init__(None, None, None, None)
    self.UpdateFromCredentials()

We can get away with passing four Nones to the superclass constructor, as it only has four positional arguments: client_id, client_secret, scope,  and user_agent. Three of those have equivalents on the OAuth2Credentials object, but there is no place for scope because that part of the token exchange handled elsewhere (OAuth2WebServerFlow) in the google-api-python-client library.

  def UpdateFromCredentials(self):
    self.client_id = self.credentials.client_id
    self.client_secret = self.credentials.client_secret
    self.user_agent = self.credentials.user_agent
    ...

Similarly, the OAuth2Credentials object only implements the refresh part of the OAuth 2.0 flow, so only has the token URI, hence auth_uri, revoke_uri and redirect_uri will not be set either. However, the token URI and the token data are the same for both.

    ...
    self.token_uri = self.credentials.token_uri
    self.access_token = self.credentials.access_token
    self.refresh_token = self.credentials.refresh_token
    ...

Finally, we copy the extra fields which may be set outside of a constructor:

    ...
    self.token_expiry = self.credentials.token_expiry
    self._invalid = self.credentials.invalid

Since OAuth2Credentials doesn't deal with all parts of the OAuth 2.0 process, we disable those methods from OAuth2Token that do.

  def generate_authorize_url(self, *args, **kwargs): raise NotImplementedError
  def get_access_token(self, *args, **kwargs): raise NotImplementedError
  def revoke(self, *args, **kwargs): raise NotImplementedError
  def _extract_tokens(self, *args, **kwargs): raise NotImplementedError

Finally, the last method which needs to be implemented is _refresh, which should refresh the OAuth2Credentials object and then update the current GData token after the refresh. Instead of using the passed in request object, we use one from httplib2 as we mentioned in imports.

  def _refresh(self, unused_request):
    self.credentials._refresh(httplib2.Http().request)
    self.UpdateFromCredentials()

After refreshing the OAuth2Credentials object, we can update the current token using the same method called in the constructor.

Using this class, we can simultaneously call a discovery-based API and a GData API:

from apiclient.discovery import build
from gdata.contacts.client import ContactsClient

service = build('calendar', 'v3', developerKey='...')

class MainHandler(webapp2.RequestHandler):
  @decorator.oauth_required
  def get(self):
    auth_token = OAuth2TokenFromCredentials(decorator.credentials)
    contacts_client = ContactsClient()
    auth_token.authorize(contacts_client)
    contacts = contacts_client.get_contacts()
    ...
    events = service.events().list(calendarId='primary').execute(
        http=decorator.http())
    ...

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