嵌套for循环中的打印（2d列表）的不稳定行为python（在循环内交换列表项）

Question

我尝试在两个嵌套for循环中尝试将1和2和2交换为2d列表中的1。我还在两个循环之前和之后显示列表 -

def train(cls, isSomethingTrue): #isSomethingTrue is always false
    print('before {0}'.format(cls.xtrain))
    for i in range(len(cls.xtrain)):
        print('before {0}'.format(cls.xtrain[i]))
        cls.ytrain.append([1, 0])
        if(not isSomethingTrue):
            for j in range(len(cls.xtrain[i])):
                if cls.xtrain[i][j] == 1:
                    cls.xtrain[i][j] = 2
                    continue
                elif cls.xtrain[i][j] == 2:
                    cls.xtrain[i][j] = 1
                    continue
        print('after {0}'.format(cls.xtrain[i]))
    print('after {0}'.format(cls.xtrain))

问题是外部循环只显示要交换的偶数索引（内部循环按预期工作）。怎么可能？

以下是截图 -