Carpet pythons are usually much more docile than the more arboreal tree boas/pythons. Carpets are the closest thing you can affordably get that has arboreal tendencies.

Looking back over the years, one of my favorite boid snakes was a carpet python that I rescued when he was young. So laid back as an adult. Not huge either, which is a plus IMO. And they easily handle husbandry mistakes, which is another plus.

Babies can be nippy, as is true for many snakes. (I have a Children's python that was the devil when tiny and is tame as can be as an adult with no attempt on my part to "tame" her.)
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Mark