# decompose world_matrix's components
orig_loc, orig_rot, orig_scale = obj.matrix_world.decompose()
#create the translation vector
translation =  mathutils.Vector((2,1,-1))
new_loc = orig_loc + translation
orig_loc_mat = Matrix.Translation(new_loc)
orig_rot_mat = orig_rot.to_matrix().to_4x4()
orig_scale_mat = Matrix.Scale(orig_scale[0],4,(sqrt(2),0,0)) *
Matrix.Scale(orig_scale[1],4,(0,1,0)) *
Matrix.Scale(orig_scale[2],4(0,0,sqrt(2)))

obj.matrix_world = orig_loc_mat * rot_mat * orig_rot_mat *
orig_scale_mat