code: def _on_access_token(self, future, response):
          
          content = escape.json_decode(response.body)
          if 'error' in content:
              LOGGER.error('Error fetching access token: %s', content['error'])
              future.set_exception(auth.AuthError('Github auth error: %s' %
                                                  str(content['error'])))
              return
          callback = self.async_callback(self._on_github_user, future,
                                         content['access_token'])
          self.github_request('user', callback, content['access_token'])
docs: Invoked as a callback when GitHub has returned a response to the
          access token request.
  
          :param method future: The callback method to pass along
          :param tornado.httpclient.HTTPResponse response: The HTTP response