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Sorry for later. A long time ago, I had saved this problem .

It just need to open terminal and input this

$ sudo update-alternatives --config python
有 2 个候选项可用于替换 python (提供 /usr/bin/python)。

  选择       路径            优先级  状态
------------------------------------------------------------
  0            /usr/bin/python3   150       自动模式
* 1            /usr/bin/python2   100       手动模式
  2            /usr/bin/python3   150       手动模式
要维持当前值[*]请按<回车键>，或者键入选择的编号：

Next you need to chose python2, just it looks like, push the number 2 & Enter. Everything is OK.