ubuntu16.04修改Python默认版本
Ubuntu16.04 默认python版本是Python2，但也可以修改默认的Python版本
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 100
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 150
备注：输入--install时，需注意是两个连续的‘-’；fyy@ubuntu:~$ python
Python 3.5.2 (default, Nov 19 2016, 06:48:10)
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>如果想要重新设置为python2,可以执行一下命令：
sudo update-alternatives –config python
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
-----------------------------------------------------------
* 0 /usr/bin/python2 100 auto mode
1 /usr/bin/python2 100 manual mode
2 /usr/bin/python3 150 manual mode
Press to keep the current choice[*], or type selection number: 0（输入对应的选择编号，此处我选择的是0）