[dev env] set up virtualenv for python, ubuntu

Summary of setting-up virtualenv

1. preparation

(make sure python installed in your ubuntu system, the installed packages may be found at /usr/bin/python2.7 or /usr/bin/python3.5)

$ pip install virtualenv
$ mkdir ~/py_venv && cd ~/py_venv

2. create venv

$ virtualenv -p /usr/bin/python2.7 --no-site-packages py2.7venv

(if python3.5 is expected for your venv, change to /usr/bin/python3.5, and give the venv a name e.g py3.5venv)

3. activate venv

$ source py2.7venv/bin/activate
(py2.7venv) $ 

4. check venv

$ python -V

5. deactivate venv

(py2.7venv) $ deactivate