Permanently add a directory to PYTHONPATH?

Question

Whenever I use sys.path.append, the new directory will be added. However, once I close python, the list will revert to the previous (default?) values. How do I

Answer 1

I added permanently in Windows Vista, Python 3.5

System > Control Panel > Advanced system settings > Advanced (tap) Environment Variables > System variables > (if you don't see PYTHONPATH in Variable column) (click) New > Variable name: PYTHONPATH > Variable value:

Please, write the directory in the Variable value. It is details of Blue Peppers' answer.