def METHOD_NAME(self) -> None:
    """Test handler for new subscriptions to creators."""
    self.login(self.USER_EMAIL)
    csrf_token = self.get_new_csrf_token()
    payload = {
        'creator_username': self.EDITOR_USERNAME
    }
    # Test that the subscriber ID is added to the list of subscribers
    # of the creator and the creator ID is added to the list of
    # subscriptions of the user.
    self.post_json(
        feconf.SUBSCRIBE_URL_PREFIX, payload,
        csrf_token=csrf_token)
    self.assertEqual(subscription_services.get_all_subscribers_of_creator(
        self.editor_id), [self.user_id])
    self.assertEqual(
        subscription_services.get_all_creators_subscribed_to(
            self.user_id), [self.editor_id])
    # Subscribing again, has no effect.
    self.post_json(
        feconf.SUBSCRIBE_URL_PREFIX, payload,
        csrf_token=csrf_token)
    self.assertEqual(subscription_services.get_all_subscribers_of_creator(
        self.editor_id), [self.user_id])
    self.assertEqual(
        subscription_services.get_all_creators_subscribed_to(
            self.user_id), [self.editor_id])
    self.logout()
    # Test another user subscription.
    self.login(self.USER2_EMAIL)
    csrf_token = self.get_new_csrf_token()
    self.post_json(
        feconf.SUBSCRIBE_URL_PREFIX, payload,
        csrf_token=csrf_token)
    self.assertEqual(subscription_services.get_all_subscribers_of_creator(
        self.editor_id), [self.user_id, self.user_id_2])
    self.assertEqual(
        subscription_services.get_all_creators_subscribed_to(
            self.user_id_2), [self.editor_id])
    self.logout()