与类指定一个创建对象的模版很相像，注释类型指定创建注释实例的模版。
Annotation types specify a template to create annotation instances, much the same way classes specify a template to create objects.
实例是由类“制造”的Python对象；类充当实例的一种模版。
An instance is a Python object that was "manufactured" by a class; the class ACTS as a sort of template for the instance.
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