你每次都是把b添加进去，相当于a[0]=b,a[1]=b ... a[n]=b，所以a中的每个元素都是相同的
假设b=[0,1,2],a.append(b)和a.append([0,1,2])是不一样的,看下面的例子
>>> b=[0,1,2]
>>> b
[0, 1, 2]
>>> a=[]
>>> a.append(b)
>>> a.append([0,1,2])
>>> a
[[0, 1, 2], [0, 1, 2]]
>>> b.append(3)
>>> a
[[0, 1, 2, 3], [0, 1, 2]]
>>>
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