A plural class name looks strange (I think it's the first time I see one in the CPython codebase). How about another name? (DistributionPool, DistributionMap, WorkingSet etc.).
Sure, WorkingSet is nice, it's the name used in setuptools,
A WorkingSet and a DistributionDirectories (or whatever it gets named to) are different things though, no? A WorkingSet is "a collection of active distributions", where each distribution might come from different distribution directories: http://peak.telecommunity.com/DevCenter/PkgResources#workingset-objects Where as DistributionDirectories is a dictionary of locations where distributions are installed. The WorkingSet may be comprised of distributions from several different locations, and each location may contain the same or different versions of the same distribution. (as far as I understand things ...) I can't really think of a better name for a dict of distribution locations ... but then I'm not averse to a pluralized class name. Overall though, I think PEP 376 is starting to look very good!