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