版本须知
ubuntu18.04 安装3个python版本，系统已自带python3.6.9和2.7.15，目前的旧链接指向python2，并且2和3产生了冲突，此时我们设置python3.6.9为默认即可
注意事项
切记不可随意删除系统自带python版本，因为系统有很多软件依赖着
删除旧版本python的软链接指向
1.查看python现有信息
root@chenwx:/usr/bin# ls /usr/bin/python* -lh
lrwxrwxrwx 1 root root 24 Jun 9 14:45 /usr/bin/python -> /etc/alternatives/python
lrwxrwxrwx 1 root root 9 Apr 16 2018 /usr/bin/python2 -> python2.7
-rwxr-xr-x 1 root root 3.5M Apr 16 01:20 /usr/bin/python2.7
lrwxrwxrwx 1 root root 9 Oct 25 2018 /usr/bin/python3 -> python3.6
-rwxr-xr-x 2 root root 4.4M Apr 18 09:56 /usr/bin/python3.6
lrwxrwxrwx 1 root root 34 Jun 9 14:41 /usr/bin/python368 -> /usr/local/python3.6.9/bin/python3
-rwxr-xr-x 2 root root 4.4M Apr 18 09:56 /usr/bin/python3.6m
-rwxr-xr-x 1 root root 1018 Oct 29 2017 /usr/bin/python3-jsondiff
-rwxr-xr-x 1 root root 3.6K Oct 29 2017 /usr/bin/python3-jsonpatch
-rwxr-xr-x 1 root root 1.4K May 2 2016 /usr/bin/python3-jsonpointer
-rwxr-xr-x 1 root root 398 Nov 16 2017 /usr/bin/python3-jsonschema
lrwxrwxrwx 1 root root 10 Oct 25 2018 /usr/bin/python3m -> python3.6m
2.重命名旧的python软链
mv /usr/bin/python /usr/bin/python.bak
mv /usr/bin/pip /usr/bin/pip.bak
3.建立python新软链和pip软链
ln -s /usr/local/python369/bin/python3 /usr/bin/python
ln -s /usr/local/python369/bin/pip3 /usr/bin/pip
4.测试版本
root@chenwx:/usr/bin# python
Python 3.6.9 (default, Jun 9 2020, 14:37:48)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
更新整个版本的python信息
我们可以使用 update-alternatives 来为整个系统更改Python 版本。以 root 身份登录（冒险，建议用普通用户外加sudo权限），首先罗列出所有可用的python 替代版本信息：
#update-alternatives --list python
update-alternatives: error: no alternatives for python
如果出现以上所示的错误信息，则表示 Python 的替代版本尚未被update-alternatives 命令识别。想解决这个问题，我们需要更新一下替代列表，将python2.7 和 python3.6放入其中。
root@chenwx:/usr/bin# update-alternatives --install /usr/bin/python python /usr/bin/python2 100
update-alternatives: using /usr/bin/python2 to provide /usr/bin/python (python) in auto mode
root@chenwx:/usr/bin# update-alternatives --install /usr/bin/python python /usr/bin/python3 110
update-alternatives: using /usr/bin/python3 to provide /usr/bin/python (python) in auto mode
root@chenwx:/usr/bin# update-alternatives --install /usr/bin/python python /usr/local/python3.6.9/bin/python3 150
update-alternatives: using /usr/local/python3.6.9/bin/python3 to provide /usr/bin/python (python) in auto mode
-install 选项使用了多个参数用于创建符号链接。最后一个参数指定了此选项的优先级，如果我们没有手动来设置替代选项，那么具有最高优先级的选项就会被选中，操作上直接回车即可
root@chenwx:/usr/bin# sudo update-alternatives --config python
There are 3 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3 110 auto mode
1 /usr/bin/python2 100 manual mode
2 /usr/bin/python3 110 manual mode
参考文献
参考文献1：设置python的文件依赖
参考文献2：ros环境依赖于python2.7环境
