While testing software to play music from a computer on my network, I have discovered Elisa, which is a wonderful piece of software built in Python and zope.interface. The package manager helpfully installed a python-zopeinterface package, which turned out to break one of the apps I've been working on (one of the packages that is used depends on a more recent version of zope.interface and breaks with a missing object import). Adding an explicit dependency on zope.interface>=3.5 didn't help either. The egg was installed and a reference to it was inserted in the generated script wrapper for the buildout's bin folder, but the system zope.interface was found.

The solution that I have found was to make sure the zope.interface dependency is listed in the last position in the install_requires section of setup.py. This has the effect of placing the zope.interface egg path first in the generated script, and thus solving the problem.

UPDATE: On another project I'm working on, this solution didn't work. Buildout would complain about a version conflict and would drop the building process. The solution was to setup a separate virtualenv bootstrapped with --no-site-packages and use the python from that virtualenv to bootstrap the buildout environment. I think it's a bug in zc.buildout, as it should have obeyed the versions section of buildout, plus the explicit dependency in install_requires of my package setup.py