Custom pointer property is read only?

Coding Python Support
1

Pretty sure i’m doing something wrong here, seems a bit strange to have a custom property default to read only (what would be the point in creating it if you can never set it to something other than the default?)

here’s a bare bones test script that can be run in a scene with a default cube:

import bpy
from bpy.props import CollectionProperty, PointerProperty, StringProperty

class CUSTOM_test(bpy.types.PropertyGroup):
    my_attr: StringProperty()
    
bpy.utils.register_class(CUSTOM_test)
bpy.types.Scene.my_test = CollectionProperty(type=CUSTOM_test)
bpy.types.Object.my_pointer = PointerProperty(type=CUSTOM_test)

bpy.context.scene.my_test.add()

# AttributeError:
bpy.context.active_object.my_pointer = bpy.context.scene.my_test[0]

which results in the following exception:
AttributeError: bpy_struct: attribute "my_pointer" from "Object" is read-only

So what’s the solution here? I have a custom PropertyGroup stored on the Scene that I need to be able to keep track of on individual objects.

nb: cross-posted on devtalk

1 Like
2

It’s read-only in the context you’re accessing it I would assume… When are you trying to set the pointer?

3

it’s read only any time I try to set it, regardless. but for the sake of example, the script in the original post is just a fire and forget script that can be run at any time and it happens then.

4

Oh right I see. Yes when you declare a pointer property to a PropertyGroup you are actually creating a reference to that prop group. So your my_pointer will have a .my_attr property. You can’t change the base reference by assignment.

If you want to reference a specific collection, you need to use an int on the object and store the collections index or use named keys.

So you could change the above code to:

bpy.types.Object.my_pointer = IntProperty()
bpy.context.scene.my_test.add()
idx = len(bpy.context.scene.my_test) - 1

bpy.context.active_object.my_pointer = idx

# Reference collection via
bpy.context.scene.my_test[bpy.context.active_object.my_pointer]
5

interesting… seems semantically incorrect to have a pointer to a custom type that doesn’t actually point to a reference of said type. that’s the opposite of how a PointerProperty to a built-in type works, eg: bpy.types.Material, etc. In your example, if the index changes, the references are still pointing to the old index, which is precisely why you’d want a pointer to it.

Oh well, I guess that’s just how things work and I’ll work around it. thanks!

6

Yes, its not very intuitive. Especially since the pointer property normally works as you would expect…

Basically you’re creating a read-only pointer to set instance of the property group assigned for that object. I think the issue is that there isn’t another way to assign the group and in most cases the current behaviour is probably what is desired.

7

Met this issue too. Solution by @netherby works but it involves some more management to make sure either not to mess with my_test order of elements or to add additional checks to make sure is up to date - for example after removing item from collection all indices that were used in other objects has to be updated.

Isn’t it just a bug after all?