How to create an in-memory zip file with directories without touching the disk?

In a python web application, I'm packaging up some stuff in a zip-file. I want to do this completely on the fly, in memory, without touching the disk. This goes fine using ZipFile.writestr as long as I'm creating a flat directory structure, but how do I create directories inside the zip?

I'm using python2.4.

http://docs.python.org/library/zipfile.html