Dropbox API - check_and_refresh_access_token

The check_and_refresh_access_token method doesn't actually return a token value; it would either internally update the access token in the client, or otherwise raise an exception, so you don't need to check the return value.

Also, you don't actually need to call check_and_refresh_access_token yourself at all. The Dropbox client will automatically call check_and_refresh_access_token itself as needed. You just need to set up the Dropbox object with your refresh token, app key, and app secret as you have and use that. It will automatically handle the refresh process for you internally.