Question

我尝试了一些代码但似乎会导致问题：

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)

然后我尝试：

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

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

这段代码怎么了？为什么不工作？有没有办法让它发挥作用？如果不是，我如何轻松透明地对最终用户缓存对象？

Answer 1

如果您想操纵创作，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__：

>>> 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

创建对象后，无法真正更改创建对象的实例。将self设置为其他内容时，您所做的只是更改变量指向的引用，因此实际对象不会受到影响。

这也解释了title属性不存在的原因。您在更改本地self变量后立即返回，从而阻止当前实例初始化title属性（更不用说此时self不会指向右侧实例）。

所以基本上，你不能在初始化期间（__init__）更改对象，因为它已经创建并分配给变量。像a = Page('test')这样的构造函数调用实际上与：

相同

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

正如您所看到的，首先调用__new__类构造函数，而这实际上是谁负责创建实例。因此，您可以覆盖类“__new__方法来操纵对象创建。

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self变量指向该函数中的内容。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?

要做你想做的事，你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

Python对象缓存

3 个答案: