There should be a big disclaimer here, that these setup.py calls are not guaranteed, and not something to build a long-term project against.  

Metadata 2.1 will try to lay down an official PEP-backed set of interfaces (http://legacy.python.org/dev/peps/pep-0426/#metabuild-system), that alternative build systems can bank on.  Up until that point, it's code at your own risk.