compression - python zipfile module doesn't seem to be compressing my files

Question

I made a little helper function:

import zipfile

def main(archive_list=[],zfilename='default.zip'):
    print zfilename
    zout = zipfile.ZipFile(zfilename, "w")
    for fname in archive_list:
        print "writing: ", fname
        zout.write(fname)
    zout.close()

if __name__ == '__main__':
    main()  

The problem is that all my files are NOT being COMPRESSED! The files are the same size and, effectively, just the extension is being change to ".zip" (from ".xls" in this case).

I'm running python 2.5 on winXP sp2.

Answer 1

This is because ZipFile requires you to specify the compression method. If you don't specify it, it assumes the compression method to be zipfile.ZIP_STORED, which only stores the files without compressing them. You need to specify the method to be zipfile.ZIP_DEFLATED. You will need to have the zlib module installed for this (it is usually installed by default).

import zipfile

def main(archive_list=[],zfilename='default.zip'):
    print zfilename
    zout = zipfile.ZipFile(zfilename, "w", zipfile.ZIP_DEFLATED) # <--- this is the change you need to make
    for fname in archive_list:
        print "writing: ", fname
        zout.write(fname)
    zout.close()

if __name__ == '__main__':
    main()  

Update: As per the documentation (python 3.7), value for 'compression' argument should be specified to override the default, which is ZIP_STORED. The available options are ZIP_DEFLATED, ZIP_BZIP2 or ZIP_LZMA and the corresponding libraries zlib, bz2 or lzma should be available.