I believe that's because gzip never operates over directories, it acts as a compression algorithm unlike zip and tar where we could compress directories. python's implementation of gzip is to operate on files. However recursive traversal of a directory tree is easy if we look at the os.walk call.

(I haven't tested this)

def gunzip(file_path,output_path):
    with gzip.open(file_path,"rb") as f_in, open(output_path,"wb") as f_out:
        shutil.copyfileobj(f_in, f_out)

def recurse_and_gunzip(root):
    walker = os.walk(root)
    for root,dirs,files in walker:
        for f in files:
            if fnmatch.fnmatch(f,"*.gz"):
                gunzip(f,f.replace(".gz",""))

import gzip
import shutil
with gzip.open('file.txt.gz', 'rb') as f_in:
    with open('file.txt', 'wb') as f_out:
        shutil.copyfileobj(f_in, f_out)

The code to compress a folder in to tar file is:

import tarfile

tar = tarfile.open("TarName.tar.gz", "w:gz")
tar.add("folder/location", arcname="TarName")
tar.close()

It works for me. Hope that works for you too.

You can try couple of changes:

1. in the subprocess, use 'gunzip' Unix command rather than 'zcat'
2. place the 'gunzip' command in a shell script file, e.g. bash shell. subprocess.call() the script file instead of the command directly. This may be helpful if you need to do additional os level manipulations such as file copies or move to differently locations etc. Make sure to set the shell script file as executable with 'chmod' on the command line.

Good luck.