[novice] getting python scripts working

[art3mis]

In my attempt at solving a pull focus issue I sourced the following script here on odforce.

 

I then paste into a Python Script digital asset node. But how to get it to run/compile? Should it run automatically?

#!/usr/bin/env hython
_version = 0.11
'''
Inital Author: "SYmek" from od[force] community.
Edited by: Farsheed Ashouri
Thanks to everyone who have helped.
For more information: http://forums.odforce.net/index.php?/topic/9725-creating-a-new-python-script-for-automating-camera-dof-setup-solved/
'''
import hou
def computeFocus():
    if hou.selectedNodes():
        for node in hou.selectedNodes():
#            print node.path()
            if node.type().name() == "cam":
                camera = node
#               else: target = node
                target = hou.node("/obj").createNode("null","DOF_Controler")
                theExp='vlength(vtorigin("%s", "%s"))'
        
                try:
                    camera.parm("focus").setExpression(theExp % (camera.path(),target.path()))
#                   cT = camera.worldTransform().extractTranslates()
#                   tT = target.worldTransform().extractTranslates()
#                   try: camera.parm("focus").set(cT.distanceTo(tT))
                    print "Done."
                except: print "Did you actually select any camera?"     

            else:
                print "Please select a camera and a target."
#if __name__ == "__main__":
computeFocus()