New ball python aggression?

Earlier today i purchased a young ball python from a pet store who claimed that he was very nice and docile. When i came home i decide to try to stroke him in his new enclosure. I simply gave him a soft stroke when he lunged at my hand (I was able to get it out in time). Why is he behaving like this? What can i do to solve the problem? How can i tell that my snake is POd at me? P.S. he did not hiss at all before he tried to bite me and i even held him at the pet store.