Your usage of os.sep is incorrect, both when reading and writing 
directories.

Zip files are (more-or-less) platform independent. The specification 
*requires* forward slashes in paths [1], and the zipfile module 
already writes them that way. Checking for os.sep is wrong - at least 
on Windows. 

I've never encountered malformed entries of that kind (like "directory
\" instead of "directory/") but if you want to suport such beasts, 
check for "/" *and* os.sep explicitely.

[1] See APPNOTE.TXT (there is a link near the top of zipfile.py)