查看目前python版本：python --version

python --version
Python 2.7.17

查看本系统中，python都有哪些版本：update-alternatives --list python

update-alternatives --list python
/etc/alternatives/python
/opt/python37/bin/python3
/opt/python37/bin/python3.7
/usr/bin/python2.7
/usr/bin/python3
/usr/bin/python3.6

我的机器里显示如上所示。

如果设置ubuntu每次启动后的默认python，则需要设置各个python版本的优先级。我想设置默认为3.7，那么3.7的优先级（Priority）设置比2.7的数字大

sudo update-alternatives --install /usr/bin/python python /opt/python37/bin/python3.7 200

sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 100

此时，重启系统后，python版本为 3.7.
另外：
在不重启系统的情况下，切换本次python版本，命令为：sudo update-alternatives --config python
如下在
Press <enter> to keep the current choice[*], or type selection number： 时输入数字3，再回车即可

sudo update-alternatives --config python
There are 6 choices for the alternative python (providing /usr/bin/python).

  Selection    Path                         Priority   Status
------------------------------------------------------------
  0            /opt/python37/bin/python3     200       auto mode
  1            /etc/alternatives/python      100       manual mode
  2            /opt/python37/bin/python3     200       manual mode
  3            /opt/python37/bin/python3.7   200       manual mode
* 4            /usr/bin/python2.7            100       manual mode
  5            /usr/bin/python3              150       manual mode
  6            /usr/bin/python3.6            50        manual mode

Press <enter> to keep the current choice[*], or type selection number: 3
update-alternatives: using /opt/python37/bin/python3.7 to provide /usr/bin/python (python) in manual mode

经过上面的选择之后，查看本次python版本

python --version
Python 3.7.4