Flyweight模式，顾名思义，就是共享元数据，在python这个动态语言中可以提高程序性能和效率，比如从一个文件读取很多的字符串，而这些字符串必定重复，所以可以使用这个模式将其存在一个pool中
普通青年版，看了Gof，可能就会有这样基础的一段代码:
class Spam(object):
def __init__(self, a, b):
self.a = a
self.b = b
class SpamFactory(object):
def __init__(self):
self.__instances = dict()
def get_instance(self, a, b):
# 在实例化后生成一个字典，当新的元祖对不存在就缓存起来
if (a, b) not in self.__instances:
self.__instances[(a, b)] = Spam(a, b)
return self.__instances[(a, b)]
# 这个和上面的意思完全一样， 这是为了在最后断言下2个工厂缓存的数据是不是能共享
class Egg(object):
def __init__(self, x, y):
self.x = x
self.y = y
class EggFactory(object):
def __init__(self):
self.__instances = dict()
def get_instance(self, x, y):
if (x, y) not in self.__instances:
self.__instances[(x, y)] = Egg(x, y)
return self.__instances[(x, y)]
spamFactory = SpamFactory()
eggFactory = EggFactory()
assert spamFactory.get_instance(1, 2) is spamFactory.get_instance(1, 2)
assert eggFactory.get_instance('a', 'b') is eggFactory.get_instance('a', 'b')
# spamFactory存储的这个元祖和eggFactory存储的不是一个东西
assert spamFactory.get_instance(1, 2) is not eggFactory.get_instance(1, 2)
上面的是最简单能想到的，也是我见过用得最多的一种， 可是还有梦想青年版， 因为上面的东西是可以抽象出来的, 比如上面只能传入a,b2个，再多了就不行了，其他的风格也不行，比如我想缓存键值对，所以想起来了_args和_*kwargs
class FlyweightFactory(object):
def __init__(self, cls):
self._cls = cls
self._instances = dict()
# 使用*args, **kargs这样的方式抽象实现的cls的参数数量和类型
def get_instance(self, *args, **kargs):
# 如果键在字典中，返回这个键所对应的值。如果键不在字典中，向字典 中插入这个键
return self._instances.setdefault(
(args, tuple(kargs.items())),
self._cls(*args, **kargs))
class Spam(object):
# 我这里实现的是3个参数，这个随意
def __init__(self, a, b, c):
self.a = a
self.b = b
self.c = c
class Egg(object):
def __init__(self, x, y, z):
self.x = x
self.y = y
self.z = z
SpamFactory = FlyweightFactory(Spam)
EggFactory = FlyweightFactory(Egg)
assert SpamFactory.get_instance(1, 2, 3) is SpamFactory.get_instance(1, 2, 3)
assert EggFactory.get_instance('a', 'b', 'c') is EggFactory.get_instance('a', 'b', 'c')
assert SpamFactory.get_instance(1, 2, 3) is not EggFactory.get_instance(1, 2, 3)
然后就是我最喜欢的风格，装饰器解决这个问题，算是老土的文艺青年吧
# 这个是装饰器，主要用来将操作工厂当参数传入，拦截操作工厂的调用
class flyweight(object):
def __init__(self, cls):
self._cls = cls
self._instances = dict()
# 重载括号操作符， 你想啊，加了装饰器就会调用，也就会触发__call__
def __call__(self, *args, **kargs):
return self._instances.setdefault(
(args, tuple(kargs.items())),
self._cls(*args, **kargs))
@flyweight
class Spam(object):
def __init__(self, a, b):
self.a = a
self.b = b
@flyweight
class Egg(object):
def __init__(self, x, y):
self.x = x
self.y = y
assert Spam(1, 2) is Spam(1, 2)
assert Egg('a', 'b') is Egg('a', 'b')
assert Spam(1, 2) is not Egg(1, 2)
但是我们实在态out了，首先，没必要把装饰器搞成一个类，完全可以使用函数式编程
# instances是闭包，好懂吧
def flyweight(cls):
instances = dict()
return lambda *args, **kargs: instances.setdefault(
(args, tuple(kargs.items())),
cls(*args, **kargs))
@flyweight
class Spam(object):
def __init__(self, a, b):
self.a = a
self.b = b
@flyweight
class Egg(object):
def __init__(self, x, y):
self.x = x
self.y = y
assert Spam(1, 2) is Spam(1, 2)
assert Egg('a', 'b') is Egg('a', 'b')
assert Spam(1, 2) is not Egg(1, 2)
该是刚进城的文艺小青版版了，这里用了一个东西Mixin: 给某个具体的类一些它需要的具体功能
# 这是Mixin类，它提供了get_instance， 谁需要这个方法谁继承，不需要不继承
class FlyweightMixin(object):
_instances = dict()
@classmethod
def get_instance(cls, *args, **kargs):
return cls._instances.setdefault(
(cls, args, tuple(kargs.items())),
cls(*args, **kargs))
class Spam(FlyweightMixin):
def __init__(self, a, b):
self.a = a
self.b = b
class Egg(FlyweightMixin):
def __init__(self, x, y):
self.x = x
self.y = y
assert Spam.get_instance(1, 2) is Spam.get_instance(1, 2)
assert Egg.get_instance('a', 'b') is Egg.get_instance('a', 'b')
assert Spam.get_instance(1, 2) is not Egg.get_instance(1, 2)
唯一不爽的是调用方式：XX.get_instance(A， B)，不够高端
class FlyweightMixin(object):
_instances = dict()
def __init__(self, *args, **kargs):
# 只想被继承不想被初始化
raise NotImplementedException
# 重载实例化触发的__new__
def __new__(cls, *args, **kargs):
return cls._instances.setdefault(
(cls, args, tuple(kargs.items())),
super(type(cls), cls).__new__(cls, *args, **kargs))
class Spam(FlyweightMixin):
def __init__(self, a, b):
self.a = a
self.b = b
class Egg(FlyweightMixin):
def __init__(self, x, y):
self.x = x
self.y = y
assert Spam(1, 2) is Spam(1, 2)
assert Egg('a', 'b') is Egg('a', 'b')
assert Spam(1, 2) is not Egg(1, 2)
嗯， 这样就和以前一样好看了，可是 什么文艺青年？ 差得很远， 一个想成为文艺青年的炫技版:
@classmethod
def _get_instance(cls, *args, **kargs):
return cls.__instances.setdefault(
(args, tuple(kargs.items())),
super(type(cls), cls).__new__(*args, **kargs))
# 其实绕了个圈子在操作工厂实例化的时候拦截执行上面的类方法
def flyweight(decoree):
decoree.__instances = dict()
decoree.__new__ = _get_instance
return decoree
@flyweight
class Spam(object):
def __init__(self, a, b):
self.a = a
self.b = b
@flyweight
class Egg(object):
def __init__(self, x, y):
self.x = x
self.y = y
assert Spam(1, 2) is Spam(1, 2)
assert Egg('a', 'b') is Egg('a', 'b')
assert Spam(1, 2) is not Egg(1, 2)
无语的文艺，好吧，现在就大众经常看见的文艺青年(穿着文艺，其实骨子里不文艺)版
# 赋予创建类的控制权,也就是python的元类metaclass
class MetaFlyweight(type):
def __init__(cls, *args, **kargs):
type.__init__(cls, *args, **kargs)
cls.__instances = dict()
# 当你实例化cls的时候，实例的结果其实是执行_get_instance方法
cls.__new__ = cls._get_instance
def _get_instance(cls, *args, **kargs):
return cls.__instances.setdefault(
(args, tuple(kargs.items())),
super(cls, cls).__new__(*args, **kargs))
class Spam(object):
# 提供你生成这个类的模板
__metaclass__ = MetaFlyweight
def __init__(self, a, b):
self.a = a
self.b = b
class Egg(object):
__metaclass__ = MetaFlyweight
def __init__(self, x, y):
self.x = x
self.y = y
assert Spam(1, 2) is Spam(1, 2)
assert Egg('a', 'b') is Egg('a', 'b')
assert Spam(1, 2) is not Egg(1, 2)
这个当然也可以搞成类方法的风格，我只列出关键的代码
@classmethod
def _get_instance(cls, *args, **kargs):
return cls.__instances.setdefault(
(args, tuple(kargs.items())),
super(type(cls), cls).__new__(*args, **kargs))
def metaflyweight(name, parents, attrs):
cls = type(name, parents, attrs)
cls.__instances = dict()
cls.__new__ = _get_instance
return cls
好吧，终极的文艺青年版- 这是一个纯函数式使用元类的方法：
metaflyweight = lambda name, parents, attrs: type(
name,
parents,
dict(attrs.items() + [
('__instances', dict()),
('__new__', classmethod(
lambda cls, *args, **kargs: cls.__instances.setdefault(
tuple(args),
super(type(cls), cls).__new__(*args, **kargs))
)
)
])
)
class Spam(object):
__metaclass__ = metaflyweight
def __init__(self, a, b):
self.a = a
self.b = b
class Egg(object):
__metaclass__ = metaflyweight
def __init__(self, x, y):
self.x = x
self.y = y
assert Spam(1, 2) is Spam(1, 2)
assert Egg('a', 'b') is Egg('a', 'b')
assert Spam(1, 2) is not Egg(1, 2)
你到了那个境界呢？