import bpy
from mathutils import Matrix

act = bpy.context.active_object
# Parent selected objects to active, empty the inverse matrix and preserve the world matrix.
for o in bpy.context.selected_objects:
    if o==act: continue
    mat_world = o.matrix_world.copy()
    o.parent_type = 'OBJECT'
    o.parent = act
    o.matrix_parent_inverse = Matrix.Identity(4)
    o.matrix_world = mat_world