Obstacles in Moveit (Python)

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How to add obstacles to MoveIt with Python in ROS Indigo? This is what I tried, but it does not add anything:

  scene_pub = rospy.Publisher('/planning_scene', PlanningScene)
  rospy.wait_for_service('/get_planning_scene', 10.0)
  get_planning_scene = rospy.ServiceProxy('/get_planning_scene', GetPlanningScene)
  request = PlanningSceneComponents(components=PlanningSceneComponents.ALLOWED_COLLISION_MATRIX)
  response = get_planning_scene(request)
  coll_mat = response.scene.allowed_collision_matrix

  print "response.scene.world.collision_objects:", response.scene.world.collision_objects

  pose_box = Pose(Point(0,0,0), Quaternion(0,0,0,1))
  box = SolidPrimitive()
  box.type = SolidPrimitive.BOX
  box.dimensions = [1,1,1]
  coll_obj = CollisionObject()
  coll_obj.header.stamp = rospy.Time(0)
  coll_obj.header.frame_id = "/base"
  coll_obj.id = "sensor_platform"
  coll_obj.operation = CollisionObject.ADD
  coll_obj.primitives.append(box)
  coll_obj.primitive_poses.append(pose_box)
  print "===== Coll Obj"
  print coll_obj

  scene_diff = PlanningScene(is_diff=True, allowed_collision_matrix = coll_mat)
  scene_diff.world.collision_objects.append(coll_obj)
  print "===== scene_diff"
  print scene_diff
  scene_pub.publish(scene_diff)
  print "Publishing msg"
  rospy.sleep(3)

  request = PlanningSceneComponents(components=16)
  response = get_planning_scene(request)
  coll_mat = response.scene.allowed_collision_matrix

  print "response.scene.world.collision_objects:", response.scene.world.collision_objects

response.scene.world.collision_objects is always an empty list.