Apply a "object pass index" to multiple objects in Blender

MultipleObjectsPassIndex.py

# COPYRIGHT: https://blenderartists.org/t/applying-object-pass-index-to-multiple-objects/562689/4

# This script apply a object pass index to multiple objects in Blender
# Select all objects to be batched first; all selected objects will be batched
import bpy

selection = bpy.context.selected_objects
passNumber = 2 #change '2' to whichever pass index you choose

bpy.ops.object.select_all(action='DESELECT')

for obj in selection:
    obj.select = True
    obj.pass_index = passNumber
    obj.select = False

for obj in selection:
    obj.select = True