supervacuo.com

Pip is pretty cool, but the "documentation" leaves a lot to be desired. Consider the mind-bendingly useful:

pip knows a bunch of stuff about revisions and stuff, so if you need to do things like install a very specific revision from a repository pip can do that too.

with no information anywhere on the page about what those various "bunch[es]" of "stuff" are. The output of pip help install is only marginally better, revealing the option:

-e VCS+REPOS_URL[@REV]#egg=PACKAGE

Happily (for me, and other users of sorl-thumbnail¹), "REV" apparently also means "branch" when installing from git. So, to save your slightly vintage applications still using ThumbnailField, install the "legacy" branch of sorl:

pip install -e git+https://github.com/sorl/sorl-thumbnail.git@legacy#egg=sorl

Now, back to hacking on InformAction I go...