Python automate Assign parent relation and keep transform

Question

I have automated the task of parenting in Blender with Python. The code affects the relation parent according to the two last characters of the names of the objects. example:

obj1.name = Cube.01
obj2.name = Suzanne.01

If obj1 is in the group "children" and obj2 is not in the group "children" : obj2 is the parent of obj1. My code works, but without keeping children's transformation. How can I do to keep the children's transformation?

import bpy

for obj1 in bpy.context.selected_objects:    
    if obj1.name in  bpy.data.groups["Children"].objects:
    a = obj1.name[-2:]

    for obj2 in bpy.context.selected_objects:
            if obj2.name not in  bpy.data.groups["Children"].objects:
                b = obj2.name[-2:]
                if a == b:                       
                   obj1.parent= obj2

Answer 1

You have to set the inverse matrix of the child to clear the initial transformation of the parent at parenting moment :

obj1.parent= obj2
obj1.matrix_parent_inverse = obj2.matrix_world.inverted()

Answer 2

An additional note to Chebhou's answer and for other community members searching for answers related to keeping the child's transform when the parent object is a Curve. The solution comes from this thread: If your parent object is a Curve, you need to set the use_path property of the curve's data to False.

curve.data.use_path = False
obj.parent = curve
obj.matrix_parent_inverse = curve.matrix_world.inverted()

Answer 3

I tried Chebhou's method but it didn't work as expected. To keep an object's position, rotation, and scale unchanged when changing its parent, you need to multiply the new parent's inverse matrix with the object's world matrix. This ensures the object maintains its world space transform after reparenting.

    # First, calculate the inverse of the parent's world matrix
    parent_inverse_world_matrix = parent.matrix_world.inverted()

    # Now, set the parent 
    obj.parent = parent
    
    obj.matrix_parent_inverse =  parent_inverse_world_matrix @ obj.matrix_world