python - 如何创建一个不重新创建具有相同输入参数的对象的类

Question

我正在尝试创建一个不会重新创建具有相同输入参数的对象的类。当我尝试使用用于创建已存在对象的相同参数实例化一个类时，我只希望我的新类返回一个指向已创建(大量创建)对象的指针。到目前为止，这是我尝试过的:

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'

这是我第一次尝试重写 __new__，我确信我的做法不对。请让我直截了当。

Answer 1

这是一个类装饰器，可以使一个类成为多元素:

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

(这是 PEP 318 中单例装饰器的一个轻微变体。)

然后，要使您的类成为多类，请使用装饰器:

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

现在，如果您使用相同的 id 实例化 MyObject，您将获得相同的实例:

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

a is b  # False
b is c  # True