I get what you're thinking, about him maybe mistaking your finger for a small mouse because everyone's saying how they don't have very good eyesight and such, but the fact of the matter is that since they don't have very good eyesight, they don't use vision so much when they hunt. You don't give off the scent of a rodent or prey item, and you don't move like one, so he doesn't go after you as food. The main triggers for a feeding response for a ball python are scent and heat signature.