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如何创建不使用相同输入参数重新创建对象的类

multiton python

2

Paul · 技术社区 · 16 年前

我正在尝试创建一个类,该类不使用相同的输入参数重新创建对象。当我试图用创建已经存在的对象所用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经创建(昂贵创建)对象的指针。这就是我迄今为止所尝试的:

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'

这是我第一次冒险 __new__ 我确信我做的不对。请把我弄直。

2 回复 | 直到 14 年前

1

7

S.Lott 16 年前

首先,在Python中使用大写类名。

第二,使用工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]

这比玩弄新对象和拥有类级对象池要简单得多。

2

12

Jerry Neumann 14 年前

下面是一个类修饰器,用于使类成为多功能:

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中的单体装饰器的一个微小变体。)

然后,要使类成为多任务类,请使用decorator:

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

现在,如果使用相同的ID实例化myObject,则会得到相同的实例:

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

a is b  # False
b is c  # True