Add file to zip, or replace file in zip

[abcd]

I have a script which zips up a directory, once it does with that
(before it closes the zip file) I want to replace a file that was added
to the zip, say "Foo.txt".

So I tried this...
[code]
z = zipfile.ZipFile("blah.zip", "w")
# zip the directory
#...
z.writestr("c:\\path\\to\\current\\foo_txt_file\\Foo.txt", "some new
data")
z.close()

All this does is add a new Foo.txt file in the zip alongside the
existing one....any suggestions?

Thanks