Ubuntu16.04安装并切换到python3.6

注意，Ubuntu16.04默认安装了Python2.7和3.5，系统自带的python千万不可卸载！

sudo apt-get install software-properties-common
sudo add-apt-repository ppa:jonathonf/python-3.6
sudo apt-get update
sudo apt-get install python3.6

执行sudo apt-get update可能会失败，尝试了多种解决办法，最终通过更换清华源解决问题：

• 先备份sources.list :
  Step 1: cd /etc/apt/
  Step 2: sudo cp sources.list sources.list.bak
• 编辑sources.list：
  Step 1: sudo gedit /etc/apt/sources.list（注：使用sudo vim /etc/apt/sources.list无法编辑，会提示只读文件）
  Step 2: 将sources.list更换为清华源：

# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted universe multiverse

# 预发布软件源，不建议启用
# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-proposed main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-proposed main restricted universe multiverse

Step 3: 保存退出后更新源使其生效
更新源：sudo apt-get update
更新软件：sudo apt-get upgrade

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.5 1
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 2

python --version    //查看当前python版本，假设现在是python2版本
echo alias python=python3 >> ~/.bashrc  //切换python默认版本类型为python3版本，从python3版本切换到python2版本只需要将命令中的3改为2
source ~/.bashrc   //令配置文件改动生效
python --version    //再次查看会发生已改为python3版本，且由于前面设置了python3.6优先级高于python3.5，此时的版本为python3.6版本