Move Bone Tail to a certain location

Question

I have an armature where Automatic IK is enabled. I want to move my bone in pose mode using python instead of grabbing it using mouse. I tried the below but it didn't work as it says tail location is a read only. How I can move the bone's tail to a certain location in the scene without stretching the bone and in pose mode not edit mode?

import bpy

ob = bpy.data.objects['Armature']

ob.pose.bones['Bone.002'].tail = bpy.context.scene.cursor_location

I've also tried selecting the bone in Pose Mode then using ops.transform.translate but it didn't move the bone to the cursor:

bpy.ops.transform.translate(value=bpy.context.scene.cursor_location, constraint_axis=(False, False, False), constraint_orientation='GLOBAL', mirror=False, proportional='DISABLED', proportional_edit_falloff='SMOOTH', proportional_size=1)

As shown below I can do this manually by selecting the bone and grabbing it to the correct position while auto-ik is enabled without resizing or stretching the bones:

Answer 1

Here is a solution using a temporary IK constraint:

The principle is to:

• Set an empty at cursor position
• Add a IK constraint to the bone targeting the empty
• Get the resulting transformation
• Remove the constraint
• Assign the previously obtained transformations

The commented code:

import bpy

#Bones and its parents
def BoneAndParents( bone ):
    yield bone
    while bone.parent:
        bone = bone.parent
        yield bone

arm = bpy.data.objects['Armature']

bpy.ops.object.mode_set(mode='OBJECT')

#Add a temporary empty
bpy.ops.object.add()
empty = bpy.context.scene.objects.active

bpy.context.scene.objects.active = arm

bpy.ops.object.mode_set(mode='POSE')

bone = arm.pose.bones['Bone.003']

#Add IK constraint and update
constraint = bone.constraints.new( 'IK' )
constraint.target = empty
bone.constraints.update()
bpy.context.scene.update() #Important: update the scene

#Get the bones and transformation due to the constraint
bones = [b for b in BoneAndParents( bone )]
transformations = [b.matrix.copy() for b in bones]

#Get rid of the temporary constraint and update
bone.constraints.remove( constraint )
bone.constraints.update()

#Assign transformation
for b, t in zip( bones, transformations ):
    b.matrix = t

#Remove temporary empty
bpy.ops.object.mode_set(mode='OBJECT')
bpy.context.scene.objects.active = empty
bpy.ops.object.delete()

#Back to pose
bpy.context.scene.objects.active = arm
bpy.ops.object.mode_set(mode='POSE')

Answer 2

Writing an updated answer for above answer ( @lemon 's answer )

# set the armature object as active object and switch to pose mode
meshname = "mesh_1"
bpy.context.view_layer.objects.active = bpy.data.objects[meshname] 
print(f"active object name = {bpy.context.active_object.name}") 
obj =bpy.context.active_object
bpy.ops.object.mode_set(mode="POSE")

# ---------
# access location of bone that you want to change
# use this location to decide the empty object's location

bone = obj.pose.bones['index_03_r']  # pose bone
location_local = bone.head.to_4d() # homogenous coordinates

# change from local coordinate to global coordinate
M_arm_to_global = obj.matrix_world.to_4x4()
location_global = M_arm_to_global @ location_local
location_global = location_global.to_3d()

# modify global location as per needed for setting the empty object
location_global[0] -= 0.1
location_global[1] -= 0.1
location_global[2] -= 0.1

# above answer has used a recursive yield. 
# i have blender's own recursive call for the parent info
# you don't need this function .
# added this for the info reason. 
def BoneAndParents(bone):
    return bone.parent_recursive

# add an empty object
bpy.ops.object.mode_set(mode="OBJECT") 
bpy.ops.object.empty_add(type='SINGLE_ARROW', align='WORLD', location=location_global, scale=(1, 1, 1))
empty = bpy.context.active_object

# set the mesh as active again
bpy.context.view_layer.objects.active = bpy.data.objects[meshname] 


# clear the parent of the bone after which you do not want IK to impact 
# bone so that arm does not move.
# you are breaking the hierarchy of the transformations 
# or using / giving less flexibilty to IK 
# I set parent to None of Hand 
bpy.ops.object.mode_set(mode="EDIT")
bpy.context.active_object.data.edit_bones['hand_r'].parent = None


# go to pose mode 
bpy.ops.object.mode_set(mode="POSE")

# move bone index_03_r to empty as target via IK
bone = obj.pose.bones['index_03_r']
constraint = bone.constraints.new("IK") 
constraint.target = empty 
bone.constraints.update()

# update API as per blender 2.82 and above
bpy.context.view_layer.update() 

# # the other lines used by lemon are not needed. 
# NOT NEEDED - FOLLOWING LINES
# ADDED FOR DISCUSSION PURPOSE AND AS ASKED BY @ROBIT in comment. 
# #Get the bones and transformation due to the constraint
# bones = [b for b in BoneAndParents( bone )]
# transformations = [b.matrix.copy() for b in bones]
# # Assign transformation
# for b, t in zip( bones, transformations ):
    # b.matrix = t