Question

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.

Answer 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.