When a ws client calls update_users(), the validate_parameters function will return all optional attributs with their default values (even if the ws client send an object without any optional attribut).
This implies that if a ws client wants to update a user, the ws client must retrieve the user with all his optional values first. Then the ws client must send the complete user object in the update_users() call (with the optional attributs set to the previously retrieved values, if these values are the same).
If the ws client doesn't send a full user object, the missing optional attributs will be overwritten by their default in the description.