A zip file which happens to let content overlay it's files. Use with a dash of StringIO for grand results.

overlayzip.py

import zipfile

class OverlayZipFile(zipfile.ZipFile):
    """ Adds a method for overlaying a zip file with a given directory, or another
    zip file. """

    def overlay(self, content):
        """ Overlays content over the zip file represented by 'self'.
        
        Receives a zip file, and write's it's entire content over the zip file which
        is represented by 'self' - overwriting any files in the current zip with the
        any files of the same name which are within the provided content zip file.
        
        """

        for filename in content.namelist():
            self.writestr(filename, content.read(filename))