How can I get pip install's -I flag to work with a requirements file?

I feel like there must be a way to do this, but for the life of me I can't figure out how: I want to run pip against a requirements file in a virtualenv so that no matter what packages are in the virtualenv before I run pip, the requirements file is totally fulfilled (including specific versions) after I run it.

The problem now is that if I have an older version of a package installed in the virtualenv than is listed in the requirements file, it complains about the version mismatch and exits (it should just update the package to the given version). The command I'm running is pip install -I -r requirements.txt and according to pip's help, -I is supposed to make pip "Ign开发者_StackOverflowore the installed packages (reinstalling instead)" but it definitely isn't doing that.

What am I missing?

(It'd be nice if pip skipped the packages that are already fulfilled too.)

I figured out what the cause of my pip problems was. Long story short, source left over in the virtualenv's build directory was causing an error that made packages upgrades fail. What I actually should have been doing was clearing out that directory (which pip doesn't always do I guess) before running the pip install and it seems to do everything I want after when paired with the --upgrade/-U flag.