def download_url(url, cache_dir):
    stem = hashlib.sha1(str(url).encode())
    filename = (cache_dir / stem.hexdigest())
    if (not filename.is_file()):
        urllib.request.urlretrieve(url, filename)
    return filename
def download_url(url, cache_dir):
    stem = hashlib.sha1(str(url).encode())
    filename = (cache_dir / stem.hexdigest())
    if (not filename.is_file()):
        urllib.request.urlretrieve(url, filename)
    return filename