OK, the concept of "abstract dependencies" makes sense, but boy is it a complicated way to fix an edge case. How many times have you had to swap out a concrete dependency vs how many times have you bashed your head against the wall about the proper way to setup a python library? I know my experience skews heavily toward the latter.

I wish python had an equivalent to npm. npm does a lot of things right: local packages by default, recursive dependency resolution, a simple declarative package specification (package.json), and the ability to run your own private npm registry. And it wraps up the logic of a bunch of tools: virtualenv + pip + (distutils / setuptools / distribute / distutils2).