I needed this for something where I wanted to send the visitor of a website a bunch of files he selected at once.
An easy way to do this would be to add the files to a zip file and then send that zip file to the user.
Unfortunately Python doesn’t have an in-memory zip file library, you can only interact with zip files on disk.

After a bit of googling around I came to this StackOverflow answer.
That worked like a charm, and here is my more reusable version:

from zipfile import ZipFile
from StringIO import StringIO

class InMemoryZipFile(object):
    def __init__(self):
        self.inMemoryOutputFile = StringIO()

    def write(self, inzipfilename, data):
        zip = ZipFile(self.inMemoryOutputFile, 'a')
        zip.writestr(inzipfilename, data)

    def read(self):
        return self.inMemoryOutputFile.getvalue()

    def writetofile(self, filename):
        open('out.zip', 'wb').write(self.read())

Save as “inmemoryzip.py” and import it as “inmemoryzip”.
This is quite limited compared to the standard ZipFile class, but this gets the job done for what I needed it.