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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def directory_tarball(dirname):
 """Create a tarball of a complex directory, avoiding complex secondaryFiles. Complex secondary files do not work on multiple platforms and are not portable to WDL, so for now we create a tarball that workers will unpack. """

    assert os.path.isdir(dirname), dirname
    base_dir, tarball_dir = os.path.split(dirname)
    while not os.path.exists(os.path.join(base_dir, "seq")) and base_dir and base_dir != "/":
        base_dir, extra_tarball = os.path.split(base_dir)
        tarball_dir = os.path.join(extra_tarball, tarball_dir)
    if base_dir == "/" and not os.path.exists(os.path.join(base_dir, "seq")):
        raise ValueError("Did not find relative directory to create tarball for %s" % dirname)
    tarball = os.path.join(base_dir, "%s-wf.tar.gz" % (tarball_dir.replace(os.path.sep, "--")))
    if not utils.file_exists(tarball):
        print("Preparing CWL input tarball: %s" % tarball)
        with file_transaction({}, tarball) as tx_tarball:
            with utils.chdir(base_dir):
                with tarfile.open(tx_tarball, "w:gz") as tar:
                    tar.add(tarball_dir)
    return tarball