Ubuntu16.04 預設python版本是Python2，但也可以修改預設的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 
 備註：輸入--install時，需注意是兩個連續的‘-’；

[email protected]:~$ 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.
>>>

如果想要重新設定為python2,可以執行一下命令：
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 <enter> to keep the current choice[*], or type selection number: 0（輸入對應的選擇編號，此處我選擇的是0）