What is the size of the Ball Python do you want to move in a 40 gallons tank? If he is a baby do not bother buying a 40 gallons tanks now, you will not need it before another 2 to 3 years.

Also keep in mind that 30 gallons tank have the same foot print than 40 gallons tank and they are cheaper, the 40 gallons only has more height which is not necessary a 30 gallons has more than enough height already.