Dixie was feeling grumpy a couple of weeks ago. I reached in her enclosure to do something and brushed against her skin. She hissed at me (kind of surprising, since I'd never heard either Phil or Magellan hiss at all). Needless to say, I didn't push her at that time. Be advised, though, that a snake does not need to hiss or "coil" or bend their neck into an 'S' shape to strike out. Unless their body is already 100% extended, they usually have enough "spring" behind them to strike out if they feel the need to.
Ball Pythons are notorious for being reluctant to bite, but it can happen. While their teeth are small in comparison to many animals, they have a lot of them, and they are sharp. It will depend on how hard the snake "tags" you how much it will hurt. If they just strike out and hit fast and withdraw, they may not even break the skin. If they latch on, you need to be mentally prepared to HOLD STILL. If you jerk your hand back, you'll likely cause the teeth to rip your skin instead of just putting some tiny punctures in. You may also injure your snake by pulling back quickly. Usually a snake like a BP will release fairly quicjly after biting something our size. Holding on is just too dangerous for them.