def _construct_key(previous_key, separator, new_key):
    """
    Returns the new_key if no previous key exists, otherwise concatenates
    previous key, separator, and new_key
    :param previous_key:
    :param separator:
    :param new_key:
    :return: a string if previous_key exists and simply passes through the
    new_key otherwise
    """
    if previous_key:
        return u"{}{}{}".format(previous_key, separator, new_key)
    else:
        return new_key
Returns the new_key if no previous key exists, otherwise concatenates
    previous key, separator, and new_key
    :param previous_key:
    :param separator:
    :param new_key:
    :return: a string if previous_key exists and simply passes through the
    new_key otherwise