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Using __getattribute__ to Validate

Our final variant uses the __getattribute__ catchall to intercept attribute fetches and manage them as needed. Every attribute fetch is caught here, so we test the attribute names to detect managed attributes and route all others to the superclass for normal fetch processing. This version uses the same __setattr__ to catch assignments as the prior version.

The code works very much like the __getattr__ version, so I won’t repeat the full description here. Note, though, that because every attribute fetch is routed to __getattribute__, we don’t need to mangle names to intercept them here (acct is stored as acct). On the other hand, this code must take care to route nonmanaged attribute fetches to a superclass to avoid looping.

Also notice that this version incurs extra calls for both setting and fetching unmanaged attributes (e.g., addr); if speed is paramount, this alternative may be the slowest of the bunch. For comparison purposes, this version amounts to 32 lines of code, just like the prior version:

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 __setattr__ too
        self.age  = age                          # acct not mangled: name tested
        self.addr = addr                         # addr is not managed
                                                 # remain has no data
    def __getattribute__(self, name):
        superget = object.__getattribute__             # Don't loop: one level up
        if name == 'acct':                             # On all attr fetches
            return superget(self, 'acct')[:-3] + '***'
        elif name == 'remain':
            return superget(self, 'retireage') - superget(self, 'age')
        else:
            return superget(self, name)                # name, age, addr: stored

    def __setattr__(self, name, value):
        if name == 'name':                             # On all attr assignments
            value = value.lower().replace(' ', '_')    # addr stored directly
        elif name == 'age':
            if value < 0 or value > 150:
                raise ValueError('invalid age')
        elif name == 'acct':
            value = value.replace('-', '')
            if len(value) != self.acctlen:
                raise TypeError('invald acct number')
        elif name == 'remain':
            raise TypeError('cannot set remain')
        self.__dict__[name] = value                     # Avoid loops, orig names

Be sure to study and run this section’s code on your own for more pointers on managed attribute coding techniques.