[Tips] “Set Inverse” for “Child Of” Constraint using Blender Python


2022-03-30 Blender Python TroubleShooting

This is a trouble shooting for using bpy.ops.constraint.childof_set_inverse.

Trouble

When I tried to set “Set Inverse” for “Child Of” constraint in bone and executed bpy.ops.constraint.childof_set_inverse(constraint=”Child Of”, owner=’BONE’) on Blender, the following error occurred. How can we solve it?

>>> bpy.ops.constraint.childof_set_inverse(constraint="Child Of", owner='BONE')
Traceback (most recent call last):
  File "<blender_console>", line 1, in <module>
  File "C:\Program Files\Blender Foundation\Blender 2.83\2.83\scripts\modules\bpy\ops.py", line 201, in __call__
    ret = op_call(self.idname_py(), None, kw)
RuntimeError: Operator bpy.ops.constraint.childof_set_inverse.poll() Context missing 'constraint'

Environment

Windows 10
Blender 2.83

Solution

To execute this operation, we should pass the context that contains target constraint and activate the target bone in pose mode.

import bpy

bone_name = "foot.IK.L"
bpy.ops.object.mode_set(mode='POSE', toggle=False)
pose_bone = bpy.context.active_object.pose.bones[bone_name]

# set the target constraint to the context
context_py = bpy.context.copy()
context_py["constraint"] = pose_bone.constraints["Child Of"]

# activate target bone
bpy.context.active_object.data.bones.active = pose_bone.bone 
bpy.ops.constraint.childof_set_inverse(context_py, constraint="Child Of", owner='BONE')

Reference

bpy.ops.constraint.apply
Childof_set_inverse