类与实例

定义实例

一个实例本质上是一个字典，里面包含的是：“方法名”－“函数”对。

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
        
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

定义类

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
        
        
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

栗子

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()) 

最后一句locals()返回一个当前框架下的字典。

Account类可使用下句生成：

>>> Account = make_account_class()

子类关于继承的实现：

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)


>>> CheckingAccount = make_checking_account_class() #子类的生成
>>> jack_acct = CheckingAccount['new']('Spock') #子类实例的生成
>>> jack_acct['get']('interest') #子类实例的方法调用
0.01