Reference to object as custom property

Coding Python Support
1

I’m trying to have a list of objects saved as a custom property within another object. This is what I’ve got so far:

class ContinentProps(bpy.types.PropertyGroup):
    number = bpy.props.IntProperty()
    object = bpy.props.PointerProperty(type = bpy.types.Object)

class ContinentsList(bpy.types.PropertyGroup):
    continentsList = bpy.props.CollectionProperty(type=ContinentProps)

    bpy.types.Object.Continents = bpy.props.PointerProperty(type = ContinentsList)

Then

D.objects[1].Continents.continentsList.add()
D.objects[1].Continents.continentsList[0].number = 10
D.objects[1].Continents.continentsList[0].object = D.objects[2]

For some reason number property works ok, but object property is not, I’m getting this when I try to read it:

>>> D.objects[1].Continents.continentsList[0].object
(<built-in function PointerProperty>, {'attr': 'object', 'type': <class 'bpy_types.Object'>})

What am I missing?

Thanks.

2

It’s explained here:

Basically, it’s impossible to store references to objects. You can store the object names in StringProperties however.

3

I had a feeling this is going to be an answer. Is there any event on changing name, deleting objects etc?

4

Or events on before save/after load of .blend file for that matter?

5

There are application callbacks in the docs.

I think there are no such callbacks which alert you when names change or properties change. Which is a problem in so many ways… Properties are entirely “polling” by nature. There values are only ever pulled by other pieces of code, and don’t have a signalling mechanism. They have something the Python/C binding refers to as callbacks, but they are just setters and getters. You could theoretically implement your own property, and then have some signalling of your own in setters/getters.

Another way to work around this problem is to use the in-memory addresses. For better or for worse, most blender data objects have an “as_pointer” method, which returns the in-memory address. This address does not change during the lifetime of the loaded blend data, regardless of changing the name. So you could try to keep a cache of pointers and names and thusly notice when a pointer gets a new name. If you want to totally abuse the Blender API, you can even do that check inside the event method of a modal operator…

These lack of modern paradigms is one of the reasons why I call Blender a “living fossil”. Blender is a great, mature piece of software, but some of the paradigms are just awfully dated. And it would be hard or impossible to change that, too.

6

The problem is, I want the references to persist through saved .blend file, so I guess saving address of the object won’t work.

Oh well, I’ll go with object names and restrict myself to not changing the names after object was created.

Thanks, everyone!

7

Another solution is to add a custom property “unique id” which is set once and never changed, so that it is saved in the blend file. It’s unfortunate that the Blender API never thought of that.

I have been thinking about how to store arbitrary data in blend files, when the “property system” just isn’t enough. One solution I came up with is to serialize the data to json and store it in either a Text object or a custom property. I haven’t tried it, but it should work.

8

Perhaps suggest that, unique id per object sounds reasonable and shouldn’t be too hard to implement.

That’s actually not a bad idea, I wasn’t aware python has a built-in json module!

9

You might find it interesting to store data (possibly JSON-encoded) as a Blender Text datablock. If you prefix the name with a dot, it will be treated as hidden (not show in UI, but still accessible via bpy.data.texts)