Ball Python Keeps snapping at me HELP!!

[lintz69]

I just got a Ball Python at a Pet Shop. I held him for about 30 minutes at the store he was great. I got him home, and he let me take him out ok, then later that day I went to get him and he snapped at me. I moved my hand out of the way in time, but my heart was thumping. The guy I got him from said he is just getting used to his new enviroment, and is testing me. After talking to him the next day and building up my confidence for a half an hour I FINALLY picked him up, and held him for about 5 -10 minutes. Now It's time to pick him up and I am terrified again... Is this common for them to snap at me? And what can I do different? This is the first snake I have owned. Please help.