如何创建类，没有重新创建与相同的输入参数的目标

Question

我试图创建一个类，不重新创建具有相同的输入参数的对象。当我尝试实例与被用来创建一个已经存在的对象相同的参数类，我只是希望我的新类的指针返回到已创建（昂贵创建）对象。这是我到目前为止已经试过：

class myobject0(object):
# At first, I didn't realize that even already-instantiated
# objects had their __init__ called again
instances = {}
def __new__(cls,x):
    if x not in cls.instances.keys():
        cls.instances[x] = object.__new__(cls,x)
    return cls.instances[x]
def __init__(self,x):
    print 'doing something expensive'

class myobject1(object):
    # I tried to override the existing object's __init__
    # but it didnt work.
    instances = {}
    def __new__(cls,x):
        if x not in cls.instances.keys():
            cls.instances[x] = object.__new__(cls,x)
        else:
            cls.instances[x].__init__ = lambda x: None
        return cls.instances[x]
    def __init__(self,x):
        print 'doing something expensive'

class myobject2(object):
    # does what I want but is ugly
    instances = {}
    def __new__(cls,x):
        if x not in cls.instances.keys():
            cls.instances[x] = object.__new__(cls,x)
            cls.instances[x]._is_new = 1
        else:
            cls.instances[x]._is_new = 0
        return cls.instances[x]
    def __init__(self,x):
        if self._is_new:
            print 'doing something expensive'

这是我的第一次创业到压倒一切的__new__，我相信我不会去它的正确途径。设置我直，请。

Answer 1

首先，使用大写类名称在Python。

其次，使用的厂设计模式来解决这个问题。

class MyObject( object ):
    def __init__( self, args ):
        pass # Something Expensive

class MyObjectFactory( object ):
    def __init__( self ):
        self.pool = {}
    def makeMyObject( self, args ):
        if args not in self.pool:
            self.pool[args] = MyObject( args )
        return self.pool[args]

这比鬼混使用对象的新的，具有一流水平池简单得多。

Answer 2

这里的一个类装饰使一个类一个多例：

def multiton(cls):
   instances = {}
   def getinstance(id):
      if id not in instances:
         instances[id] = cls(id)
      return instances[id]  
   return getinstance

（这是从PEP 318单身装饰的轻微变体。）

然后，让你的类多例，使用装饰：

@multiton
class MyObject( object ):
   def __init__( self, arg):
      self.id = arg
      # other expensive stuff

现在，如果实例MyObject来使用相同的ID，你会得到相同的实例：

a = MyObject(1)
b = MyObject(2)
c = MyObject(2)

a is b  # False
b is c  # True