When a target is part of a mutable object (an attribute reference, subscription or slicing), the mutable object must ultimately perform the assignment and decide about its validity, and may raise an exception if the assignment is unacceptable.

• 当目标是一个可变对象的一部分(属性引用，下标，片断)的时候，该可变对象必须最终执行该赋值，决定其有效性，并且如果该赋值不可接受可能会抛出一个例外。