2.6.1   Instances

We begin with instances. An instance has named attributes, such as the balance of an account, which can be set and retrieved. We implement an instance using a dispatch dictionary that responds to messages that "get" and "set" attribute values. Attributes themselves are stored in a local dictionary called attributes.

As we have seen previously in this chapter, dictionaries themselves are abstract data types. We implemented dictionaries with lists, we implemented lists with pairs, and we implemented pairs with functions. As we implement an object system in terms of dictionaries, keep in mind that we could just as well be implementing objects using functions alone.

To begin our implementation, we assume that we have a class implementation that can look up any names that are not part of the instance. We pass in a class to make_instance as the parameter cls.

>>> def make_instance(cls):
        """Return a new object instance, which is a dispatch dictionary."""
        def get_value(name):
            if name in attributes:
                return attributes[name]
            else:
                value = cls['get'](name)
                return bind_method(value, instance)
        def set_value(name, value):
            attributes[name] = value
        attributes = {}
        instance = {'get': get_value, 'set': set_value}
        return instance

The instance is a dispatch dictionary that responds to the messages get and set. The set message corresponds to attribute assignment in Python's object system: all assigned attributes are stored directly within the object's local attribute dictionary. In get, if name does not appear in the local attributes dictionary, then it is looked up in the class. If the value returned by cls is a function, it must be bound to the instance.

Bound method values. The get_value function in make_instance finds a named attribute in its class with get, then calls bind_method. Binding a method only applies to function values, and it creates a bound method value from a function value by inserting the instance as the first argument:

>>> def bind_method(value, instance):
        """Return a bound method if value is callable, or value otherwise."""
        if callable(value):
            def method(*args):
                return value(instance, *args)
            return method
        else:
            return value

When a method is called, the first parameter self will be bound to the value of instance by this definition.

2.6.2   Classes

A class is also an object, both in Python's object system and the system we are implementing here. For simplicity, we say that classes do not themselves have a class. (In Python, classes do have classes; almost all classes share the same class, called type.) A class can respond to get and set messages, as well as the new message:

>>> def make_class(attributes, base_class=None):
        """Return a new class, which is a dispatch dictionary."""
        def get_value(name):
            if name in attributes:
                return attributes[name]
            elif base_class is not None:
                return base_class['get'](name)
        def set_value(name, value):
            attributes[name] = value
        def new(*args):
            return init_instance(cls, *args)
        cls = {'get': get_value, 'set': set_value, 'new': new}
        return cls

Unlike an instance, the get function for classes does not query its class when an attribute is not found, but instead queries its base_class. No method binding is required for classes.

Initialization. The new function in make_class calls init_instance, which first makes a new instance, then invokes a method called __init__.

>>> def init_instance(cls, *args):
        """Return a new object with type cls, initialized with args."""
        instance = make_instance(cls)
        init = cls['get']('__init__')
        if init:
            init(instance, *args)
        return instance

This final function completes our object system. We now have instances, which set locally but fall back to their classes on get. After an instance looks up a name in its class, it binds itself to function values to create methods. Finally, classes can create new instances, and they apply their __init__ constructor function immediately after instance creation.

In this object system, the only function that should be called by the user is make_class. All other functionality is enabled through message passing. Similarly, Python's object system is invoked via the class statement, and all of its other functionality is enabled through dot expressions and calls to classes.

2.6.3   Using Implemented Objects

We now return to use the bank account example from the previous section. Using our implemented object system, we will create an Account class, a CheckingAccount subclass, and an instance of each.

The Account class is created through a make_account_class function, which has structure similar to a class statement in Python, but concludes with a call to make_class.

>>> def make_account_class():
        """Return the Account class, which has deposit and withdraw methods."""
        interest = 0.02
        def __init__(self, account_holder):
            self['set']('holder', account_holder)
            self['set']('balance', 0)
        def deposit(self, amount):
            """Increase the account balance by amount and return the new balance."""
            new_balance = self['get']('balance') + amount
            self['set']('balance', new_balance)
            return self['get']('balance')
        def withdraw(self, amount):
            """Decrease the account balance by amount and return the new balance."""
            balance = self['get']('balance')
            if amount > balance:
                return 'Insufficient funds'
            self['set']('balance', balance - amount)
            return self['get']('balance')
        return make_class(locals())

The final call to locals returns a dictionary with string keys that contains the name-value bindings in the current local frame.

The Account class is finally instantiated via assignment.

>>> Account = make_account_class()

Then, an account instance is created via the new message, which requires a name to go with the newly created account.

>>> kirk_account = Account['new']('Kirk')

Then, get messages passed to kirk_account retrieve properties and methods. Methods can be called to update the balance of the account.

>>> kirk_account['get']('holder')
'Kirk'
>>> kirk_account['get']('interest')
0.02
>>> kirk_account['get']('deposit')(20)
20
>>> kirk_account['get']('withdraw')(5)
15

As with the Python object system, setting an attribute of an instance does not change the corresponding attribute of its class.

>>> kirk_account['set']('interest', 0.04)
>>> Account['get']('interest')
0.02

Inheritance. We can create a subclass CheckingAccount by overloading a subset of the class attributes. In this case, we change the withdraw method to impose a fee, and we reduce the interest rate.

>>> def make_checking_account_class():
        """Return the CheckingAccount class, which imposes a $1 withdrawal fee."""
        interest = 0.01
        withdraw_fee = 1
        def withdraw(self, amount):
            fee = self['get']('withdraw_fee')
            return Account['get']('withdraw')(self, amount + fee)
        return make_class(locals(), Account)

In this implementation, we call the withdraw function of the base class Account from the withdraw function of the subclass, as we would in Python's built-in object system. We can create the subclass itself and an instance, as before.

>>> CheckingAccount = make_checking_account_class()
>>> jack_acct = CheckingAccount['new']('Spock')

Deposits behave identically, as does the constructor function. withdrawals impose the $1 fee from the specialized withdraw method, and interest has the new lower value from CheckingAccount.

>>> jack_acct['get']('interest')
0.01
>>> jack_acct['get']('deposit')(20)
20
>>> jack_acct['get']('withdraw')(5)
14

Our object system built upon dictionaries is quite similar in implementation to the built-in object system in Python. In Python, an instance of any user-defined class has a special attribute __dict__ that stores the local instance attributes for that object in a dictionary, much like our attributes dictionary. Python differs because it distinguishes certain special methods that interact with built-in functions to ensure that those functions behave correctly for arguments of many different types. Functions that operate on different types are the subject of the next section.