Virtual environment (virtualenv) helps us have different python versions & different python libraries and their versions installed on our system in isolated environment (environment in this case is basically a folder/directory) without affecting our global system python version & its libraries.
Installing python virtual environment is not too difficult on your system.
Just open the terminal ( Press ctrl+alt+t in Ubuntu. You may have to check the key combination for your distro) and fire the command
pip install virtualenv
virtualenv -p /usr/bin/python2.7 my_virtual_env_folder_name
ls -lsh my_virtual_env_folder_name/
pip install requests
pip install lxml