Child Of's set inverse without using bpy.ops

Question

I have the following code:

import bpy

obj = bpy.data.objects["Cube"]
target = bpy.data.objects["Lamp"]
const = obj.constraints.new(type='CHILD_OF')
const.name = "Custom Const"
const.target = target

When I run this code in the default scene Cube jumps into the Lamp's position and gets Lamp's orientation (just as expected). Now I have to set inverse for the Cube. I could use something like:

C = bpy.context.copy()
C["constraint"] = const
bpy.ops.constraint.childof_set_inverse(C, constraint=const.name, owner='OBJECT')

But it looks hackish and not always works for multiple objects (at least for me). E.g.:

import bpy

objects = [bpy.data.objects["Cube"], bpy.data.objects["Camera"]]
target = bpy.data.objects["Lamp"]

for obj in objects:
    const = obj.constraints.new(type='CHILD_OF')
    const.name = "Custom Const"
    const.target = target
    C = bpy.context.copy()
    C["constraint"] = const
    bpy.ops.constraint.childof_set_inverse(C, constraint=const.name, owner='OBJECT')

So the main question is how to set inverse for a Child Of constraint without using bpy.ops? I understand that I've to do something with object's matrices but I've no idea what.

Answer 1

Try:

const.inverse_matrix = target.matrix_world.inverted()   
obj.update_tag({'OBJECT'})
bpy.context.scene.update()

This does not show up immediatly. You have to update the view / tag the region for redraw.

Answer 2

If the target object is an armature and a pose bone is selected then const.inverse_matrix looks like this:

matrix_final = target.matrix_world * target_bone.matrix
const.inverse_matrix = matrix_final.inverted()