ubuntu use python in virtualenv

when you have installed python3.5 or python2.7, then copy codes following to use python3.5 or python2.7 in virtualenv.

1. install pip

$ sudo apt-get install python-pip

2. use pip to install virtualenv

$ pip install virtualenv

3. make a directory noly for virtualenv environment

 $ mkdir virtualenvs

4. Install python3.5 virtualenv, "--no-site-packages" means to make site-packages for python3.5 in virtualenv are separated from site-packages out of the virtualenv.

$ cd virtualenvs/
$ virtualenv --no-site-packages --distribute -p /usr/bin/python3.5 py3.5

5. activate py3.5

$ source py3.5/bin/activate

6. check the python version at present.

$ which python

7. you can also deactivate py3.5

$ deactivate

8. you may have python2.7 virtualenv as well. Use code like py3.5.

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

If you have two python version (python2 and python3),you can use sudo apt install virtualenv
Also, you can use source py3.5/bin/activate and deactivate to change python versions.