python2和python3共存方法

拿到安装包，安装python3

centos: 

sudo yum install python36

ubuntu:

sudo add-apt-repository ppa:deadsnakes/ppa
sudo apt-get update
sudo apt-get install python3.6

安装pip并升级到最新版本:

python -m pip install --upgrade pip

python3 -m pip install --upgrade pip

如果发现python3安装没有带pip:

python3 -m ensurepip

检查python2和python3是否分开：

pip -V

pip 19.2.1 from /usr/lib/python2.7/site-packages/pip (python 2.7)

pip3 -V

pip 19.2.1 from /usr/local/lib/python3.6/site-packages/pip (python 3.6)

升级pip后，找不到pip:

whereis pip

使用软链接：

sudo ln -s /usr/bin/pip2.7  /usr/bin/pip