享元模式

享元模式通过共享对象来实现减少内存的使用。

import weakref

class Link(object):
    def __init__(self, ref, text, image_path=None):
        self.ref = ref
        if image_path:
            self.image = BrowserImage(image_path)
        else:
            self.image = None
        self.text = text

    def __str__(self):
        if not self.image:
            return "<Link (%s)>" % self.text
        else:
            return "<Link (%s,%s)>" % (self.text, str(self.image))


class BrowserImage(object):
    _resources = weakref.WeakValueDictionary()
    
    def __new__(cls, location):
        image = BrowserImage._resources.get(location, None)
        if not image:
            image = object.__new__(cls)
            BrowserImage._resources[location] = image
            image.__init(location)
        return image

    def __init(self, location):
        self.location = location
        # self.content = load picture into memory

    def __str__(self,):
        return "<BrowserImage(%s)>" % self.location

icon = Link("www.pythonunlocked.com",
            "python unlocked book",
            "http://pythonunlocked.com/media/logo.png")

footer_icon = Link("www.pythonunlocked.com/#bottom",
                   "unlocked series python book",
                   "http://pythonunlocked.com/media/logo.png")

twitter_top_header_icon = Link("www.twitter.com/pythonunlocked",
                               "python unlocked twitter link",
                               "http://pythonunlocked.com/media/logo.png")

print icon
print footer_icon
print twitter_top_header_icon

在上面的例子中，使用Link类来保存链接数据，Browser使用这些链接，可能存在很多链接引用同一张图片，如果每个图片都保存下来，会很占用内存。所以这里使用享元模式，只用创建一个BrowserImage对象即可。

上述代码 的输出结果为：

<Link (python unlocked book,<BrowserImage(http://pythonunlocked.com/media/logo.png)>)>
<Link (unlocked series python book,<BrowserImage(http://pythonunlocked.com/media/logo.png)>)>
<Link (python unlocked twitter link,<BrowserImage(http://pythonunlocked.com/media/logo.png)>)>