Pythonのディスクリプタのサンプル。

discriptor.py

class Thumb(object):
    def __init__(self, path):
        self.url = 'http://hoge.com' + path
        self.path = path

class ThumbDiscriptor(object):
    BASE_PATH  = '/img/{0}/{0}_{1}.{2}'

    def __init__(self, name):
        self.name = name
        
    def __get__(self, model, klass):
        extension = model.image.split('.')[-1]
        tmp_path = self.BASE_PATH.format(model.id, self.name, extension)
        return Thumb(tmp_path)

class Hoge(object):
    id = 10
    image = '/img/hoge.jpg'
    thumb = ThumbDiscriptor('main_480x360')

c = Hoge()

print c.thumb.url
print c.thumb.path