def _make_digest(k, **kwargs):
    """
    Creates a digest suitable for use within an :class:`phyles.FSCache`
    object from the key object `k`.

    >>> adict = {'a' : {'b':1}, 'f': []}
    >>> make_digest(adict)
    'a2VKynHgDrUIm17r6BQ5QcA5XVmqpNBmiKbZ9kTu0A'
    """
    result = list()
    result_dict = _make_digest_dict(k, **kwargs)
    if result_dict is None:
        return 'default'
    else: 
        for (key, h) in sorted(result_dict.items()):
            result.append('{}_{}'.format(key, h))
        return '.'.join(result)