This is especially an option for the larger candle holders. Take into account already when building the candlesticks. You can then later choose to put a candle or a light bulb in the candlestick.
The only difference between a pure candle holder and a candle holder that keeps both options open is the bottom plate made of mirror glass. The lamp cable is simply inserted through an opening in the bottom of the lamp. This allows you to switch from candlelight to electric light and vice versa at any time.