@staticmethod
def __cache_get_default_tag__(clf, x_data, n_grams=None):
    """Create and return a default cache tag."""
    n_grams = n_grams or len(clf.__max_fr__[0]) if len(clf.__max_fr__
        ) > 0 else 1
    return '%s_%s[%d-data]' % (clf.get_name(), 'words' if n_grams == 1 else
        '%d-grams' % n_grams, len(x_data))
