heroku/heroku-buildpack-python — pull request #54 by DrMeers (2012-07-20 06:11:44+00:00)
Title: Allow ${ENVIRONMENT_VARIABLES} within requirements.txt

So that sensitive/environment-specific credentials don't need to be stored in the (potentially shared/public) repository itself.

e.g. `-e git+https://${GITHUB_USER}:${GITHUB_PASSWORD}@github.com/my/private-repo.git`

I can't imagine `${` would appear in a requirements file ordinarily? If I'm mistaken, we could add an opt-in setting to enable the functionality instead of detecting those characters.

Ideally we'd allow SSH keys to be used for pip requirement retrieval instead/as well, but I'm finding this sufficient for now.