<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _on_access_token(self, future, response):
 """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 """

        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'])