Quickly switch Python run version, how to choose Python version

You will be faced with the choice of python2.x or python3.x when learning python.
I downloaded different versions of Python in different locations on my computer, and when I was learning, no matter which version I needed to run, it was OK to switch versions quickly.
Change the settings inside the environment variable , such as:

My installation location is:

D:\Program Files\python36;d:\program files\python36\scripts;

C:\Python27; C:\Python27\Scripts;
Two versions can be switched freely
See if the success is as follows:

Corresponds to Python version 2.7

Corresponds to Python version 3.6

Quickly switch Python run version, how to choose Python version