There is no quick way to learn Jython API but to experiment with it. The easiest way is to start with Jytutor extension for XL Deploy.
Now you can also use the code snippet for exposing jython/python context in XL Deploy environment by running it directly in Jytutor
Here’s how you can go ahead with that
Download the Jytutor extension referring to the Jytutor Blog or from the following link https://github.com/xebialabs-community/xld-jytutor-plugin/releases
Shutdown your XL Deploy server and drop the extension jar file under XLD_HOME/plugins folder
Restart the server and log back in
You should be able to see Jytutor as the last option in top menu. Click to open that
Now refer to the code snippet in XL Deploy Variables Demystified : Part 2
Once you copy the snippet in Commands section and submit it, this is how it looks.