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

This should uninstall anything not in requirements.txt:

pip freeze | grep -v -f requirements.txt - | grep -v '^#' | xargs pip uninstall -y

Although this won't work quite right with packages installed with -e, i.e. from a git repository or similar. To skip those, just filter out packages starting with the -e flag:

pip freeze | grep -v -f requirements.txt - | grep -v '^#' | grep -v '^-e ' | xargs pip uninstall -y

Then, obviously:

pip install -r requirements.txt

Update for 2016: You probably don't really want to actually use the above approach, though. Check out pip-tools and pip-sync which accomplish what you are probably looking to do in a much more robust way.

https://github.com/nvie/pip-tools

Update for May, 2016:

You can now also use pip uninstall -r requirements.txt, however this accomplishes basically the opposite - it uninstalls everything in requirements.txt

Update for May, 2019:

Check out pipenv or Poetry. A lot has happened in the world of package management that makes this sort of question a bit obsolete. I'm actually still quite happily using pip-tools, though.

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 <file>    Uninstall all the packages listed in the given requirements file.  This option can be
                              used multiple times.
...