No way... The only issue would be if your dog has high prey drive for snakes! LOL. Terriers and cattledogs often inherently have high prey drive, and might try to bite/chase down your snake. I don't know your dog.
The ball python certainly wouldn't be able to harm or care about your dog, unless you're intentionally doing something completely ridiculous like... hitting your dog with your ball python? lol...
On the flip side... it would take a GIANT reticulated python or something to be able to pose a danger to your 20 lb dog.