def _find_key(my_dict, value):
    """
    :param my_dict: defaultdict
    :param value: a value you want to find
    :return:
    key of that value
    """
    my_dict_val = my_dict.values()
    my_dict_item = my_dict.items()

    idx = my_dict_val.index(value)
    key = my_dict_item[idx][0]

    return key