refrence: https://virtualenv.pypa.io/en/latest/userguide.html#usage
1. install
#pip install virtualenv
2. create a isolated env
#cd ~ #mkdir -p py_env/dj_1.4 #virtualenv py_env/dj_1.4
3. active env
#. bin/activate
or
#source bin/activate
4. deactivate
#deactivate
5.pip install demo
Reference: https://pip.pypa.io/en/latest/reference/pip_install.html#git
#pip install git+https://github.com/django/django.git@1.7b3
virtualenv (isolated virtual environment) for python runtime
原文：http://www.cnblogs.com/BugQiang/p/4709400.html