To summarize the high-level API needed: install from an index (argument is a version requirement as string), install archive file, install from directory (containing setup.cfg or setup.py).

When #8668 is implemented, an “editable” option will be nice too.  (Need to check pip doc and use cases to see if only the install from directory should get it.)

For the implementation, as discussed on IRC we want to go with classes to represent the various kinds of installable things (requirement, path to a file, path to a dir) and one or more high-level functions to wrap them for common cases (see Nick’s good article http://www.boredomandlaziness.org/2012/05/djangos-cbvs-are-not-mistake-but.html ).  I would prefer having two functions, one that takes a version req and another one that takes a path; it’s easy to distinguish between file (archive) and directory but I don’t like guessing between name and path (see the not-cool code in pysetup actions).