def get_formid(spray_operator, spray_date, spray_operator_code=None):
    """
    Returns a string with 'DAY.MONTH.SPRAY_OPERATOR_CODE' from a spray_operator
    and spray_date.
    """
    return "%s.%s" % (
        spray_date.strftime("%d.%m"),
        spray_operator.code if spray_operator else spray_operator_code,
    )