类实例实现,初始化实例-来自sicp python

这段代码有点棘手,所以您会发现其中一些问题并不奇怪。要理解它,你需要理解 closures . 在python中有一些关于闭包的信息 this answer .

init_instance 使用创建新实例 instance = make_instance(cls) ,然后它查找 init 方法 cls ,如果它存在,它称之为 初始化 方法与新实例以及所传递的任何内容 args . 既不 make_instance 也不 初始化实例 修改 CLS 字典,或 attributes 字典被传递给 make_class 什么时候 CLS 被创造。实际发生的是 制作实例 创建新的 属性 它创建的实例的dict get 和 set 实例DICT中的函数可以引用。

你的 make_my_class 定义没有多大意义。它有多余的 pass 声明,以及 make_class({'__init__': __init__}) 因为你还没有定义 __init__ 在任何地方,都需要一个函数来初始化类实例。

这是你的代码的修改版本。我创造了一个简单的 爱因斯坦 功能 my_class ,并添加了几个 print 这样我们就能知道代码在做什么。

def hexid(obj):
    return hex(id(obj))

def make_instance(cls): # good with this
    """ Return a new object instance, which is a dispatch dictionary """
    def get_value(name):
        print('INSTANCE GET_VALUE', name, 'from', hexid(attributes))
        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 = {'test': 'Default Test'}
    print('Created instance attributes', hexid(attributes))
    instance = {'get': get_value, 'set': set_value}
    return instance

def bind_method(value, instance): # good with this
    """ 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

def make_class(attributes, base_class=None): 
    """ Return a new class, which is a dispatch dictionary. """
    def get_value(name):
        print('\nCLASS GET_VALUE', name, 'from', hexid(attributes))
        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)

    print('Creating class with attributes', hexid(attributes))
    cls = {'get': get_value, 'set': set_value, 'new': new}
    return cls

def init_instance(cls, *args): # problem here
    """ Return a new object with type cls, initialized with args """
    instance = make_instance(cls)
    init = cls['get']('__init__')
    if init:
        print('Calling init of', hexid(cls), 'on', hexid(instance), 'with', args)
        init(instance, *args)          #No return here
    return instance

def make_my_class(): # define a custom class
    # Create a simple __init__ for the class
    def __init__(inst, *args):
        print('INIT', hexid(inst), args)
        inst['set']('data', args)

    # return a dict that implements class
    return make_class({'__init__': __init__})

# test

#create a class
my_class = make_my_class()

#create some class instances
jim = my_class['new']('Jim')
jim['set']('test', 'Hello')

fred = my_class['new']('Fred') 

print('CLASS', hexid(my_class))
print('\nINSTANCE', hexid(jim))
print(jim['get']('data'))
print(jim['get']('test'))

print('\nINSTANCE', hexid(fred))
print(fred['get']('data'))
print(fred['get']('test'))

输出

Creating class with attributes 0xb71e67d4
Created instance attributes 0xb71373ec

CLASS GET_VALUE __init__ from 0xb71e67d4
Calling init of 0xb7137414 on 0xb71373c4 with ('Jim',)
INIT 0xb71373c4 ('Jim',)
Created instance attributes 0xb7137374

CLASS GET_VALUE __init__ from 0xb71e67d4
Calling init of 0xb7137414 on 0xb713734c with ('Fred',)
INIT 0xb713734c ('Fred',)
CLASS 0xb7137414

INSTANCE 0xb71373c4
INSTANCE GET_VALUE data from 0xb71373ec
('Jim',)
INSTANCE GET_VALUE test from 0xb71373ec
Hello

INSTANCE 0xb713734c
INSTANCE GET_VALUE data from 0xb7137374
('Fred',)
INSTANCE GET_VALUE test from 0xb7137374
Default Test