Green tree pythons, as a general rule, do not make the best pets if you expect to handle them a great deal. However, captive bred GTPs are fairly calm and will tolerate moderate handling for short periods.
Green tree pythons tend to get stressed easily when outside of their cages, or when something “invades” their cage area. This stress is often misinterpreted as aggression. When a snake is stressed, they only have two options: become defensive, or run away. In my experience, most GTPs will choose the aggression option when they are in their cages, and the run option when outside the cage. As a keeper you need to be prepared to read and understand the body language of a GTP when they are calm and relaxed, and when they are becoming stressed. Also there is a right way and a wrong way to handle them. They should always be lifted from underneath, and never grabbed fully around their bodies. Picking them up from above or grasping them fully around the body are very threatening actions, and they react accordingly.
The green tree pythons that I keep all have different personalities. Some are calmer than others, and I have only owned one that could not be free handled. While they often become more comfortable with handling as they get older, additional handling appears to have little impact on how snappy they are.
Another point is that green tree pythons are nocturnal, and when they are naturally awake at night time they are focused on hunting and feeding. This is NOT the time to try handling. Their instincts are on “high alert” for any movement that might be food. Reptiles are opportunistic feeders, and don’t really respond to hunger like mammals do. So even though you just fed your animal, it will still eat when food is present, and your hand may appear to look like food.