Reallocates a dead pointer Object

Question

Consider the following situation:
Suppose you store the Cube in a global variable and prints the object's position every 2 seconds. Assume you never delete the object.

import bpy

ob = [bpy.data.objects["Cube"]]

def in_2_seconds():
    print(ob[0].location)
    return 2.0

bpy.app.timers.register(in_2_seconds)

Scripts will not work when you perform undo/redo operations in the UI.

Traceback (most recent call last):
  File "\Text", line 6, in in_2_seconds
ReferenceError: StructRNA of type Object has been removed

The simple fix should be:

import bpy

ob = [bpy.data.objects["Cube"]]
name = [ob[0].name]

def in_2_seconds():
    try: ob[0].name
    except:
        print("---- Object is Dead ----")
        ob[0] = bpy.data.objects[name[0]]
        name[0] = ob[0].name

    print(ob[0].location)
    return 2.0

bpy.app.timers.register(in_2_seconds)

But it's not always correct, like when you rename the Cube then undo once and redo once.
Then what is the correct way to fix that?

Answer 1

Use bpy.types.PointerProperty assigned to bpy.types.Scene.

bpy.props.PointerProperty(type = bpy.types.ID)

Can use any ID, not affected by renaming and it is safe for Undo/Redo.

For many objects you can use bpy.props.CollectionProperty that as type uses a sub-class of bpy.types.PropertyGroup that contains the pointer property.

import bpy


if not hasattr(bpy.types.Scene, 'my_objects'):
    
    class Object_Pointer(bpy.types.PropertyGroup):
        target: bpy.props.PointerProperty(type = bpy.types.ID)
    
    bpy.utils.register_class(Object_Pointer)

    bpy.types.Scene.my_objects = bpy.props.CollectionProperty(type=Object_Pointer)


if not hasattr(bpy.types.WindowManager, 'my_data'):
    bpy.types.WindowManager.my_data = {}


bpy.context.scene.my_objects.clear()

for object in bpy.data.objects:
    item = bpy.context.scene.my_objects.add()
    item.target = object


def in_2_seconds():

    print("Count:", len(bpy.context.scene.my_objects))

    for index, item in enumerate(bpy.context.scene.my_objects):
        object = item.target
        if object:
            print(index, object.name_full, object.location)
        else:
            print(index, "Looks like the object has been deleted.")

    return 2.0


wm = bpy.data.window_managers[0]

prev_func = wm.my_data.get('prev_func')
if prev_func and bpy.app.timers.is_registered(prev_func):
    bpy.app.timers.unregister(prev_func)
    
bpy.app.timers.register(in_2_seconds, persistent = True)

wm.my_data['prev_func'] = in_2_seconds