def make_targz(output_filename, source_dir):

    with tarfile.open(output_filename, "w:gz") as tar:

        tar.add(source_dir, arcname=os.path.basename(source_dir))

def make_targz_one_by_one(output_filename, source_dir):

    tar = tarfile.open(output_filename,"w:gz")

    for root,dir,files in os.walk(source_dir):

        for file in files:

            pathfile = os.path.join(root, file)

            tar.add(pathfile)

    tar.close()

make_targz("test.tar.gz","a.txt") 

#or

make_targz_one_by_one("test.tar.gz","a.txt")