class CardHolder:
    acctlen = 8                               # Class data
    retireage = 59.5

    def __init__(self, acct, name, age, addr):
        self.acct = acct                         # Instance data
        self.name = name                         # These trigger __set__ calls too
        self.age  = age                       # __X not needed: in descriptor
        self.addr = addr                         # addr is not managed
                                                 # remain has no data
    class Name:
        def __get__(self, instance, owner):   # Class names: CardHolder locals
            return self.name
        def __set__(self, instance, value):
            value = value.lower().replace(' ', '_')
            self.name = value
    name = Name()

    class Age:
        def __get__(self, instance, owner):
            return self.age                          # Use descriptor data
        def __set__(self, instance, value):
            if value < 0 or value > 150:
                raise ValueError('invalid age')
            else:
                self.age = value
    age = Age()

    class Acct:
        def __get__(self, instance, owner):
            return self.acct[:-3] + '***'
        def __set__(self, instance, value):
            value = value.replace('-', '')
            if len(value) != instance.acctlen:        # Use instance class data
                raise TypeError('invald acct number')
            else:
                self.acct = value
    acct = Acct()

    class Remain:
        def __get__(self, instance, owner):
            return instance.retireage - instance.age    # Triggers Age.__get__
        def __set__(self, instance, value):
            raise TypeError('cannot set remain')        # Else set allowed here
    remain = Remain()


bob = CardHolder('1234-5678', 'Bob Smith', 40, '123 main st')
print(bob.acct, bob.name, bob.age, bob.remain, bob.addr, sep=' / ')
bob.name = 'Bob Q. Smith'
bob.age  = 50
bob.acct = '23-45-67-89'
print(bob.acct, bob.name, bob.age, bob.remain, bob.addr, sep=' / ')

sue = CardHolder('5678-12-34', 'Sue Jones', 35, '124 main st')
print(sue.acct, sue.name, sue.age, sue.remain, sue.addr, sep=' / ')
try:
    sue.age = 200
except:
    print('Bad age for Sue')

try:
    sue.remain = 5
except:
    print("Can't set sue.remain")

try:
    sue.acct = '1234567'
except:
    print('Bad acct for Sue')


print("bob's name:", bob.name)