def drop_cycle(edge, max_length=20):
    """
    删除列表中形成的小闭环
    @edge: 原始顶点id
    @max_length: 容许闭环的最小长度
    return: 输出删除小闭环后的列表
    """
    drop_list = []
    drop_count = 0
    for i, item in enumerate(edge):
        if item not in drop_list:
            drop_list.append(item)
        else:
            last_index = len(drop_list) - 1 - drop_list[::-1].index(item)
            if i - last_index - drop_count < max_length:
                drop_count += len(drop_list[last_index:])
                drop_list = drop_list[:last_index+1]
            else:
                drop_list.append(item)
    edge = np.asarray(drop_list)
    # TODO 去掉首尾构成的闭环  如： [956 1035 1538 ...... 2028 1035 952 956] ==> 1035->952->956->1035
    return edge

Test

a = [2, 3, 1, 2, 5, 8, 1, 6, 1, 2, 1, 3, 4, 1, 2]
print(drop_cycle(a, 3))        # [2 3 1 2 5 8 1 3 4 1 2]