It is important to always read your snake's behavior.  Ball pythons are usually very docile and calm.  It is still important to observe your snake especially when removing your snake from it's enclousure.  When you first attempt to remove your snake, try and gently touch your snake on the back to ensure that your snake is aware of your presence.  Approach your snake carefully and quickly.  Hesitation while attempting to pick up your snake can cause your snake to become anxious and afraid.  You always want to pick up your snake by gently lifting it from behind, not in the snake's face.  Some ball pythons can become defensive from not being handled by their previous owner.  Other ball pythons can be very food oriented and always in search of food.  Being mistaken as food is one of the most common reasons for being bitten by a ball python.  If your ball python is in an S position from the head and neck, the python is in a striking position.  Again, do not approach the python from the front or in it's face.  The python can also hiss at you by breathing in and then blowing the air out quickly.  This is another defense mechanism.  It is a gesture meant to intimidate the person or animal that is approaching.  This is just a bluff.