def is_valid(id: str):
    """
    Calculates whether an ID number if valid
    """
    if len(id) != 9:
        return False
    _sum = 0
    for i in range(8):
        if i % 2 == 0:
            weight = 1
        else:
            weight = 2
        value = int(id[i]) * weight
        while value > 9:
            value = value // 10 + value % 10
        _sum += value

    if _sum % 10 == 0:
        return int(id[8]) == 0
    else:
        return int(id[8]) == 10 - (_sum % 10)