django 中的延迟加载技术，python中的lazy技术

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说起lazy_object,首先想到的是django orm中的query_set、fn.Stream这两个类。

query_set只在需要数据库中的数据的时候才 产生db hits。Stream对象只有在用到index时才会去一次次next。

例子：

　　　f = Stream()

        fib = f << [0, 1] << iters.map(add, f, iters.drop(1, f))

　1行生成了斐波那契数列。

说明：

　　f是个lazy的对象，f首先放入了0和1，然后放入了迭代器iters.map。等f[2]的时候。就会调用1次next（iters.map(add, f, iters.drop(1, f))），在map的迭代器中，next(f)和next（drop（1，f））被放到add两端。

很高端大气上档次有木有！

这里有个django的lazy的实现代码，

　对象的

#coding: utf-8
#
class LazyProxy(object):

    def __init__(self, cls, *args, **kwargs):

        self.__dict__['_cls'] = cls
        self.__dict__['_params'] = args
        self.__dict__['_kwargs'] = kwargs

        self.__dict__["_obj"]=None

    def __getattr__(self, item):

        if self.__dict__['_obj'] is None:
            self._init_obj()

        return getattr(self.__dict__['_obj'], item)

    def __setattr__(self, key, value):

        if self.__dict__['_obj'] is None:
            self._init_obj()

        setattr(self.__dict__['_obj'], key , value)

    def _init_obj(self):

        self.__dict__['_obj']=object.__new__(self.__dict__['_cls'],
                                             *self.__dict__['_params'],
                                             **self.__dict__['_kwargs'])
        self.__dict__['_obj'].__init__(*self.__dict__['_params'],
                                       **self.__dict__['_kwargs'])

class LazyInit(object):

    def __new__(cls, *args, **kwargs):
        return LazyProxy(cls, *args, **kwargs)

class A(LazyInit):

    def __init__(self, x):

        print ("Init A")
        self.x = 14 + x

a = A(1)
print "Go"
print a.x

原理：在类的__new__方法中hook一下，使其返回lazy_proxy 的对象。然后调用__init__方法时，其实就是调用proxy的__init__方法，第一次调用时

也就是当A生成实例时，Proxy才会真正产生一个A的类，并初始化这个类，注意，在这里proxy的init中得到的cls是A而不是Lazy_Init，因为只有A（1）调用时A的__new__才会调用，虽然__new__名字的查找在LazyInit中。

函数的lazy：

"""
lazy - Decorators and utilities for lazy evaluation in Python
Alberto Bertogli (albertito@blitiri.com.ar)
"""  

class _LazyWrapper:
    """Lazy wrapper class for the decorator defined below.
    It's closely related so don't use it. 

    We don't use a new-style class, otherwise we would have to implement
    stub methods for __getattribute__, __hash__ and lots of others that
    are inherited from object by default. This works too and is simple.
    I'll deal with them when they become mandatory.
    """
    def __init__(self, f, args, kwargs):
        self._override = True
        self._isset = False
        self._value = None
        self._func = f
        self._args = args
        self._kwargs = kwargs
        self._override = False  

    def _checkset(self):
        print '', self._isset, self._value
        if not self._isset:
            self._override = True
            self._value = self._func(*self._args, **self._kwargs)
            self._isset = True
            self._checkset = lambda: True
            self._override = False  

    def __getattr__(self, name):
        print '----------getattr----',  name
        if self.__dict__['_override']:
            return self.__dict__[name]
        self._checkset()
        print '@@@@@@@@@', self._value, type(self._value), name, self._value.__getattribute__(name)
        return self._value.__getattribute__(name)  

    def __setattr__(self, name, val):
        print '----------setattr----',  name, val
        if name == '_override' or self._override:
            self.__dict__[name] = val
            return
        self._checkset()
        print ''
        setattr(self._value, name, val)
        return  

def lazy(f):
    "Lazy evaluation decorator"
    def newf(*args, **kwargs):
        return _LazyWrapper(f, args, kwargs)  

    return newf  

@lazy
def quick_exe():
    print '---------quick exe-----------'
    return 'quickquick'  

import pdb
#pdb.set_trace()  

quick_exe()
print '#####################'
print quick_exe()

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