python __getattr__

 1 class ObjectDict(dict):
 2     def __init__(self, *args, **kwargs):
 3         super(ObjectDict, self).__init__(*args, **kwargs)
 4 
 5     def __getattr__(self, name):
 6         value =  self[name]
 7         if isinstance(value, dict):
 8             value = ObjectDict(value)
 9         return value
10 
11 if __name__ == ‘__main__‘:
12     od = ObjectDict(asf={‘a‘: 1}, d=True)
13     print od.asf, od.asf.a     # {‘a‘: 1} 1
14     print od.d                 # True

 1 class WidgetShowLazyLoad(object):
 2     def fetch_complex_attr(self, attrname):
 3         ‘‘‘可能是比较耗时的操作， 比如从文件读取‘‘‘
 4         return attrname
 5 
 6     def __getattr__(self, name):
 7         if name not in self.__dict__:
 8              self.__dict__[name] = self.fetch_complex_attr(name) 
 9         return self.__dict__[name]
10 
11 if __name__ == ‘__main__‘:
12     w = WidgetShowLazyLoad()
13     print ‘before‘, w.__dict__
14     w.lazy_loaded_attr
15     print ‘after‘, w.__dict__

import functools
class lazy_attribute(object):
    """ A property that caches itself to the class object. """

    def __init__(self, func):
        functools.update_wrapper(self, func, updated=[])
        self.getter = func

    def __get__(self, obj, cls):
        value = self.getter(cls)
        setattr(cls, self.__name__, value)
        return value

class Widget(object):
    @lazy_attribute
    def complex_attr_may_not_need(clz):
        print ‘complex_attr_may_not_need is needed now‘
        return sum(i*i for i in range(1000))

if __name__ == ‘__main__‘:
    print Widget.__dict__.get(‘complex_attr_may_not_need‘)  # <__main__.lazy_attribute object at 0x02B12450>
    Widget.complex_attr_may_not_need                        # complex_attr_may_not_need is needed now
    print Widget.__dict__.get(‘complex_attr_may_not_need‘)  # 332833500

class adaptee(object):
    def foo(self):
        print ‘foo in adaptee‘
    def bar(self):
        print ‘bar in adaptee‘

class adapter(object):
    def __init__(self):
        self.adaptee = adaptee()

    def foo(self):
        print ‘foo in adapter‘
        self.adaptee.foo()

    def __getattr__(self, name):
        return getattr(self.adaptee, name)

if __name__ == ‘__main__‘:
    a = adapter()
    a.foo()
    a.bar()

class AlgoImpA(object):
    def __init__(self):
        self.obj_attr = ‘obj_attr in AlgoImpA‘

    def foo(self):
        print ‘foo in AlgoImpA‘

    def bar(self):
        print ‘bar in AlgoImpA‘

class AlgoImpB(object):
    def __init__(self):
        self.obj_attr = ‘obj_attr in AlgoImpB‘

    def foo(self):
        print ‘foo in AlgoImpB‘

    def bar(self):
        print ‘bar in AlgoImpB‘

class Algo(object):
    def __init__(self):
        self.imp_a = AlgoImpA()
        self.imp_b = AlgoImpB()
        self.cur_imp = self.imp_a

    def switch_imp(self):
        if self.cur_imp == self.imp_a:
            self.cur_imp = self.imp_b
        else:
            self.cur_imp = self.imp_a

    def __str__(self):
        return ‘Algo with imp %s‘ % str(self.cur_imp)


    def __getattr__(self, name):
        return getattr(self.cur_imp, name)


if __name__ == ‘__main__‘:
    algo = Algo()
    
    print algo
    print algo.obj_attr
    algo.foo()
    
    algo.switch_imp()
    
    print algo
    print algo.obj_attr
    algo.bar()