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()