Custom properties: updating from python and reading from driver

Question

I'm still a blender beginner, sorry in advance.

I have a python script that registers frame handlers (frame_change_post) that can update some custom properties of the objects in the scene. I can see these properties being updated when I run the animation, everything is fine so far.

Then, I have a few drivers (on shader nodes) that use these custom properties, i.e. have them as input variables. My problem is that these input variables are not updated: I can see the input values in the drivers editor as the animation runs and they do not change throughout the animation, unlike the actual custom properties. The animation itself visually confirms that the input values never change.

I have other drivers that read custom properties on the same object, but these properties are key-framed. And for those, everything works as expected.

Any help would be greatly appreciated :)

Edit: I saw somewhere that going through a Value node could help, except the driver is for the frame offset on an Image Texture node, and it is not an input so I can't plug the Value in.

Edit 2: never mind the value of the Value node with appropriate driver is not updated either :(

Answer 1

It seems your problem comes from the fact drivers won't reevaluate if they don't detect their sources to change. And it seems modifying the property in Python doesn't trigger this update, nor does view layer or depsgraph update(). What works, however, is changing any driver.

So I'm setting up a simple scene, with a left cube having a driver on Z location sin(frame/5) to just move vertically, a right cube having a driver on Z location last_z, where last_z points to a custom scene property scene["last_z"], and a cylinder, which is there just to show you don't need to modify a driver related to the animation you want to update:

Now the below code assigns updates the custom scene property, therefore animating the right cube, and updates any driver to force updating the dependency graph:

import bpy
from bpy import data as D


def frame_change_post(scene):
    scene['last_z'] = D.objects['Cube'].location.z 
    D.objects['Cylinder'].animation_data.drivers[0].driver.expression += ''
    

bpy.app.handlers.frame_change_post.clear()
bpy.app.handlers.frame_change_post.append(frame_change_post)

Result in 2 FPS to show the cubes are synchronized: