We currently check raw.info['bads'] = ... because we can do it in Info.__setitem__, but we don't check raw.info['bads'] += because this is a += operation on the bads list. One hopefully easy way to fix this would be to subclass List to check any inplace ops (append, extend, +=, ...?) to check if the channels can be added. This would require keeping a reference to the Info object, though (because you need the channel names), but that should be possible in principle I think.