Python对象缓存

Question

我尝试了一些代码，但似乎导致了问题：

class Page:
    cache = []


    """ Return cached object """
    def __getCache(self, title):
        for o in Page.cache:
            if o.__searchTerm == title or o.title == title:
                return o
        return None


    """ Initilize the class and start processing """
    def __init__(self, title, api=None):
        o = self.__getCache(title)
        if o:
            self = o
            return
        Page.cache.append(self)

        # Other init code
        self.__searchTerm = title
        self.title = self.someFunction(title)

然后我试着：

a = Page('test')
b = Page('test')

print a.title # works
print b.title # AttributeError: Page instance has no attribute 'title'

这段代码有什么问题？为什么它不能工作？有没有办法让它工作呢？如果不是，我该如何轻松、透明地访问最终用户缓存对象呢？

Answer 1

如果您想要操作创建，则需要更改__new__。

>>> class Page(object):
...     cache = []
...     """ Return cached object """
...     @classmethod
...     def __getCache(cls, title):
...         for o in Page.cache:
...             if o.__searchTerm == title or o.title == title:
...                 return o
...         return None
...     """ Initilize the class and start processing """
...     def __new__(cls, title, api=None):
...         o = cls.__getCache(title)
...         if o:
...             return o
...         page = super(Page, cls).__new__(cls)
...         cls.cache.append(page)
...         page.title = title
...         page.api = api
...         page.__searchTerm = title
...         # ...etc
...         return page
... 
>>> a = Page('test')
>>> b = Page('test')
>>> 
>>> print a.title # works
test
>>> print b.title
test
>>> 
>>> assert a is b
>>> 

编辑:使用__init__的

>>> class Page(object):
...     cache = []
...     @classmethod
...     def __getCache(cls, title):
...         """ Return cached object """
...         for o in Page.cache:
...             if o.__searchTerm == title or o.title == title:
...                 return o
...         return None
...     def __new__(cls, title, *args, **kwargs):
...         """ Initilize the class and start processing """
...         existing = cls.__getCache(title)
...         if existing:
...             return existing
...         page = super(Page, cls).__new__(cls)
...         return page
...     def __init__(self, title, api=None):
...         if self in self.cache:
...             return
...         self.cache.append(self)
...         self.title = title
...         self.api = api
...         self.__searchTerm = title
...         # ...etc
... 
>>> 
>>> a = Page('test')
>>> b = Page('test')
>>> 
>>> print a.title # works
test
>>> print b.title
test
>>> assert a is b
>>> assert a.cache is Page.cache
>>> 

Answer 2

一旦创建了对象，您就不能真正更改它的实例。当将self设置为其他值时，您所要做的就是更改变量指向的引用，这样实际的对象就不会受到影响。

这也解释了为什么title属性不在那里。一旦更改了本地self变量，就会立即返回，从而阻止当前实例初始化title属性(更不用说此时的self不会指向正确的实例)。

因此，基本上，您不能在初始化期间更改对象(在__init__中)，因为此时它已经被创建，并被分配给变量。像a = Page('test')这样的构造函数调用实际上与：

a = Page.__new__('test')
a.__init__('test')

如您所见，首先调用的是__new__类构造函数，而这实际上是负责创建实例的人。因此，您可以重写类的__new__方法来操作对象创建。

但是，通常首选的方法是创建一个简单的工厂方法，如下所示：

@classmethod
def create (cls, title, api = None):
    o = cls.__getCache(title)
    if o:
        return o
    return cls(title, api)

Answer 3

self是一个普通的局部变量，因此设置self = ..只会更改该函数中的self变量所指向的内容。它不会改变实际的对象。

请参阅：Is it safe to replace a self object by another object of the same type in a method?

要做您想做的事情，您可以使用静态函数作为factory

class Page:
    cache = []


    """ Return cached object """
    @staticmethod
    def create(title):
        for o in Page.cache:
            if o.__searchTerm == title or o.title == title:
                return o
        return Page(title)

    """ Initilize the class and start processing """
    def __init__(self, title, api=None):
        Page.cache.append(self)

        # Other init code
        self.__searchTerm = title
        self.title = title


a = Page.create('test')
b = Page.create('test')

print a.title
print b.title