A rather simple practical example I'm working on right now: moving a Django app from 1.9 to 2.0. There are a few code breaking changes in 2.0. All I need to do

A rather simple practical example I'm working on right now: moving a Django app from 1.9 to 2.0. There are a few code breaking changes in 2.0. All I need to do is create a new virtualenv with Django 2.0 and I can just switch between both versions simply by changing the environment. I may even need to change some further packages that do not work with 2.0 yet and swap them for something else. I can do that in the new env. Then, when all is working, I export a requirements.txt from my virtual env and can quickly set up the very same virtual env on my production machine.