+1 I had "tsort" in my own utilities library for so long that I thought it was in stdlib already. I found this issue after unsuccessfully searching docs.python.org. :-) I think functools will be a fine place for it. It is somewhat related to total ordering and solves the problem which is common when implementing functional mini-languages. Another possibility is shutil given that tsort is a standard POSIX command, <http://pubs.opengroup.org/onlinepubs/009695299/utilities/tsort.html>, but I think this will be too obscure.