Ball pythons can easily live in a tank half that size given that they handled and taken care of on a daily basis. While I understand the idea of giving an animal alot of space to live, this doesn't really apply to snakes, especially ball pythons. All they will do is curl up in one corner of the tank under a log and sleep there until you take them out, hence the term "ball" python.
A locking lid is a great idea. Ball pythons aren't quite as bad about escaping like the corn snakes. I would not suggest using a terrarium for a ball python. About the only kind of snake I would use a terrarium for is a arboreal snake, like a tree boa.
Ball pythons are a great beginner snake. They do have a tendency to fall ill though, so just prepared for that. Otherwise they are very friendly and easy going. I don't believe I've ever met a mean ball python.
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