changing how instances are "created"

Robert Kern rkern at ucsd.edu
Mon Jun 13 01:49:25 CEST 2005


newseater wrote:
> 
> Robert Kern wrote:
> 
>>newseater wrote:
>>
>>>Hello. I need to be able to control how objects are created. Sometimes
>>>when creating an object, i want to reuse another object instead. I've
>>>searched for factory method implementations and singleton
>>>implementations. They were too much of a hack!
>>>
>>>My first attempt of doing this was to play around with the __new__()
>>>method. But i didn't quite succed. Then i came up with making a static
>>>method which can create my instances or re-use other instances.
>>>However, the below code I cannot get to work :(
>>>
>>>class Creator
>>>	def createInstance(cls, *args, **kwargs):
>>>		anewinstance = cls.__new__(cls, *args, **kwargs)
>>>		anewinstance.__init__(*args, **kwargs)
>>>
>>>		return anewinstance
>>>	createInstance = staticmethod(createInstance)
>>
>>You want a classmethod, not a staticmethod.
> 
> why do i want that? how should the code look like? currently the
> objects fail to get initialized etc...

A staticmethod does not take a cls argument. It is essentially just a 
function that is attached to a class.

class Something(object):
     def foo(x, y, z):
         print x, y, z
     foo = staticmethod(foo)

A classmethod does that a cls argument.

class Creator(object):
     def createInstance(cls, *args, **kwds):
         pass
     createInstance = classmethod(createInstance)

As for the desired content of the classmethod, I don't care to 
speculate. I usually just use the Borg pattern or a factory or any one 
of the Singleton implementations floating around. They are no more hacky 
than this, but they have the added benefit of working.

-- 
Robert Kern
rkern at ucsd.edu

"In the fields of hell where the grass grows high
  Are the graves of dreams allowed to die."
   -- Richard Harter




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