Keep the top level folder when using zipfile to zip a directory

I'm having a hard to making zipfile to retain the structure. This is the code that I am currently using:

target_dir = "/tmp/Folder"
zip = zipfile.ZipFile('/tmp/manual.zip', 'w', zipfile.ZIP_DEFLATED)
rootlen = len(target_dir) + 1
    for base, dirs, files in os.walk(target_dir):
        for file in files:
            fn = os.path.join(base, file)
            zip.write(fn, fn[rootlen:])
zip.close()

It will make a zip the file but after I extract it, the "Folder" dir will be gone and only the files inside the Folder will be received. How do I do this? What I want to happen is that when I extract the zip, it will show as 'Folder' and it should contain the files inside.