mac中需要用到python2的环境，但之前把python2删除了，
需要重新安装：
到官网下载安装包：https://www.python.org/downloads/release/python-2711/
傻瓜式安装好后：
~/Github/ganglia-api on master ⌚ 13:54:43
$ which python
/usr/bin/python
~/Github/ganglia-api on master ⌚ 14:02:23
$ which python2
/usr/local/bin/python2
安装虚拟环境工具：
~ ⌚ 14:10:08
$ pip2 install virtualenv
无论使用pip2还是pip3安装的virtualenv都是一样的，默认情况下，pip2无法找到virtualenv
~ ⌚ 14:11:05
$ pip3 install virtualenv
Requirement already satisfied: virtualenv in /usr/local/lib/python3.7/site-packages (16.4.3)
配置python2、python3的虚拟环境目录：
~ ⌚ 14:20:21
$ virtualenv -p /usr/local/bin/python2 .py2_env
Running virtualenv with interpreter /usr/local/bin/python2
New python executable in /Users/along/.py2_env/bin/python
Installing setuptools, pip, wheel...
done.
~ ⌚ 14:20:50
$ virtualenv -p /usr/local/bin/python3 .py3_env
Running virtualenv with interpreter /usr/local/bin/python3
Using base prefix '/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7'
New python executable in /Users/along/.py3_env/bin/python3.7
Also creating executable in /Users/along/.py3_env/bin/python
Installing setuptools, pip, wheel...
done.
或：
/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages ⌚ 16:14:08
$ chmod +x virtualenv.py
/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages ⌚ 16:14:30
$ ./virtualenv.py -p /usr/local/bin/python2 ~/.py2_env
Running virtualenv with interpreter /usr/local/bin/python2
New python executable in /Users/along/.py2_env/bin/python
Installing setuptools, pip, wheel...
done.
/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages ⌚ 16:15:04
$ ./virtualenv.py -p /usr/local/bin/python3 ~/.py3_env
Running virtualenv with interpreter /usr/local/bin/python3
Using base prefix '/Library/Frameworks/Python.framework/Versions/3.7'
New python executable in /Users/along/.py3_env/bin/python3
Also creating executable in /Users/along/.py3_env/bin/python
Installing setuptools, pip, wheel...
done.
为python2安装module:
~ ⌚ 14:24:20
$ cd .py2_env/bin
~/.py2_env/bin ⌚ 14:24:27
$ source activate
(.py2_env)
~/.py2_env/bin ⌚ 14:24:55
$
(.py2_env)
~/.py2_env/bin ⌚ 14:26:54
$ pip install tornado