How can I use a pip requirements file to uninstall as well as install packages?

Question

I have a pip requirements file that changes during development.

Can pip be made to uninstall packages that do not appear in the requirement

Answer 1

You can now pass the -r requirements.txt argument to pip uninstall.

pip uninstall -r requirements.txt -y

At least as of pip 8.1.2, pip help uninstall shows:

...
Uninstall Options:
  -r, --requirement     Uninstall all the packages listed in the given requirements file.  This option can be
                              used multiple times.
...