def ISO_WEEK_YEAR(expression):
    """
    Returns the year number in ISO 8601 format.
    The year starts with the Monday of week 1 and ends with the Sunday of the last week.
    See https://docs.mongodb.com/manual/reference/operator/aggregation/isoWeekYear/
    for more details
    :param expression: expression or variable of a Date, a Timestamp, or an ObjectID
    :return: Aggregation operator
    """
    return {'$isoWeekYear': expression}