How to create a class that doesn't re-create an object with identical input parameters

问题

I am trying to create a class that doesn't re-create an object with the same input parameters. When I try to instantiate a class with the same parameters that were used to create an already-existing object, I just want my new class to return a pointer to the already-created (expensively-created) object. This is what I have tried so far:

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'

This is my first venture into overriding __new__ and I'm convinced I'm not going about it the right way. Set me straight, please.

回答1:

First, use Upper Case Class Names in Python.

Second, use a Factory design pattern to solve this problem.

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]

This is much simpler than fooling around with new and having class level pools of objects.

回答2:

Here's a class decorator to make a class a multiton:

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

(This is a slight variant of the singleton decorator from PEP 318.)

Then, to make your class a multiton, use the decorator:

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

Now, if you instantiate MyObject with the same id, you get the same instance:

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

a is b  # False
b is c  # True

来源：https://stackoverflow.com/questions/669932/how-to-create-a-class-that-doesnt-re-create-an-object-with-identical-input-para

标签

python

multiton