How to use shape key value in scripted expression driver?

Question

I have two objects, mesh A and mesh B, where mesh B has a shape key named shape-change with a value range of 0 to 1. I added a driver to the rotation Z property of mesh A which should make it rotate when the value of the shape key of mesh B changes. How do I implement the driver script?

Answer 1

A much more simple solution would be:

1. Select "object b", over shape key Value, right mouse click and Copy as New Driver

2. Select "object a", Rotation Z value and Paste Driver

3. Now change driver settings to Scripted Expression and change expression to value*2*pi

result:

Answer 2

Follow my setup: make sure object A is named mesh-obj-a and its mesh is named mesh-a and object B is named mesh-obj-b and its mesh is named mesh-b. Also make sure that object B has a shape key under Shape Keys named shape-change

Now, there's two ways to do this. The first method or easy way is to execute this python script in the Text Editor and you're done.

import bpy

obj_a = bpy.data.objects['mesh-obj-a']
obj_b = bpy.data.objects['mesh-obj-b']

drivers = obj_a.animation_data.drivers
for d in drivers:  
    obj_a.driver_remove(d.data_path, -1)

fcurve = obj_a.driver_add("rotation_euler", 2)
d = fcurve.driver

v = d.variables.new()
v.name = "keys"

target = v.targets[0]
target.id_type = 'MESH'
target.id = obj_b.data
target.data_path = "shape_keys"

d.expression = "radians(keys.key_blocks['shape-change'].value*360)"

The second method is the manual way by doing the following:

Select object A and right click its Rotation Z property in the Object Properties and select Add Driver

Click the X icon to delete the default input variable and click Add Input Variable. Change the variable name from var to keys. Then change the ID-Block type from Object to Mesh and select the mesh mesh-b

In the Expression field input the following script which converts the shape key value 0-1 to 0-360 as radians (0.0174533 is the factor for degrees to radians conversion):

keys.key_blocks['shape-change'].value*360*0.0174533

or better:

radians(keys.key_blocks['shape-change'].value*360)

In the Path field, input the value shape_keys

Now when you change the value of the shape key of object B from 0 to 1, it will drive or rotate object A on the z-axis from 0 to 360 degrees.