Get the saved access token for private resources from the session.

def _get_at_from_session(self):
        """
        Get the saved access token for private resources from the session.
        """
        try:
            return self.request.session['oauth_%s_access_token'
                                        % get_token_prefix(
                                            self.request_token_url)]
        except KeyError:
            raise OAuthError(
                _('No access token saved for "%s".')
                % get_token_prefix(self.request_token_url))