安装python，python3后设置默认顺序

详细情况见：
https://blog.csdn.net/CYM_LMY/article/details/78315139

显示Python代替版本信息

update-alternatives --list python

执行后显示

update-alternatives: error: no alternatives for python 表示Python代替版本尚未安装

执行一下命令安装

sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1

结果：update-alternatives: using /usr/bin/python2.7 to provide /usr/bin/python (python) in auto mode

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2

结果：update-alternatives: using /usr/bin/python3.5 to provide /usr/bin/python (python) in auto mode

显示Python代替版本信息

update-alternatives --list python

切换版本update-alternatives --config python

选择序号即可切换