查看可用版本
ls /usr/bin/python*
/usr/bin/python /usr/bin/python2.7 /usr/bin/python3.5 /usr/bin/python3m
/usr/bin/python2 /usr/bin/python3 /usr/bin/python3.5m
查看可替换版本信息
update-alternatives --list python
/usr/bin/python2.7
/usr/bin/python3.5
如果显示如下信息：
{%r%}
update-alternatives: error: no alternatives for python
{%endr%}
则表示Python 的替换版本尚未被update-alternatives 命令识别。想解决这个问题，我们需要更新一下替换版本，将Python2.7和python3.5放入其中。执行以下命令：
{%g%}
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2
{%endg%}
切换版本
sudo su
update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.5 2 auto mode
1 /usr/bin/python2.7 1 manual mode
2 /usr/bin/python3.5 2 manual mode
Press <enter> to keep the current choice[*], or type selection number:
输入1，则切换到python2.7
然后查看一下python版本
python --version
Python 2.7.12
参考链接：
https://www.cnblogs.com/Yanfang20180701/p/10588087.html