The table is built on a per-class basis, not for each instance of the class.
表是基于类构造，不是基于实例构造。
PE lessons are still organized on a class basis, which makes it difficult for teachers to apply flexibly differentiated teaching.
体育课组织形式还是以班级为主，使得教师不能区别对待，给 体育教学带来很大的麻烦。
You might add a check in the metaclass code to see if the attributes were already set in a parent class, but this adds plumbing and it does not give real control on a per-class basis.
也许会在元类代码中添加一个检查，以查看是否在父类中设置了这些属性，但是这样会增加负担，并且不会控制所有的类。
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