[calvins]

Pip has a constraints file now. Running

  $ pip freeze -r requirements.txt > constraints.txt

after you've installed all your packages gives you a constraints file that can be used to reinstall exactly the same versions:

  $ pip install -r requirements.txt -c constraints.txt

[jbbarth]

Didn't know that option thanks. Better than nothing, but unfortunately your environment is still subject to the remarks in my second paragraph. So pip-tools is still required if you want more guarantees.