I saw a post about speeding up the Python packaging command “pip install”, by specifying more responsive mirrors for querying and downloading packages. For my situation, a better tactic is this.

Step one: Download all your project’s dependencies into a local ‘packages’ dir, but don’t install them yet:

mkdir packages
pip install --download=packages -r requirements.txt

Step two, install from the ‘packages’ dir:

pip install --no-index --find-links=packages -r requirements.txt

(The above syntax works on pip 1.3, released yesterday. Docs for older versions of pip claim to support this, but in practice, for pip 1.2, I’ve had to use “--find-links=file://$PWD/packages”)

Step 2 works even if PyPI is unreachable. It works even if some of your dependencies are self-hosted by the authors, and that website is unreachable. It works even if the version you have pinned of one of your dependencies has been deleted by the author (some packages do this routinely after security updates.) It works even if you have no network connection at all. In short, it makes creation of your virtualenv bulletproof.

As a nice side effect, it runs really fast, because it isn’t downloading the packages across the internet, nor is it attempting to scan a remote index to check for matching or newer versions of each package. This is much quicker than just using a Pip download cache, especially for large projects with many dependencies which only change occasionally.

At Rangespan, we check the ‘packages’ directory into source control, so that once you’ve checked out a project’s repo, you have everything you need to deploy locally and run, even if you have no network. You might choose to treat ‘packages’ as ephemeral.

It was pointed out to me recently by @jezdez, Pip maintainer, this usage pattern has now been explicitly called out in the documentation, which was substantially reorganised and improved with the recent 1.3 release.