Yes it will harm your ball python to get cold. I keep lots of 60 hour heat packs on hand and if the power goes out, I would attach one of these to the top of my warm side... keeping it warm.

Then I'd cover the tub with a nice thick blanket or towels... keeping the heat in.

Since your thermometer should be battery operated, you'll be able to see if you are keeping the temps up warm enough. If it's a small heat pack... you could use two or three at one time.