Can __new__ prevent __init__ from being called?

[Michael Spencer]

Peter Hansen wrote:
> Felix Wiemann wrote:
> 
>> Sometimes (but not always) the __new__ method of one of my classes
>> returns an *existing* instance of the class.  However, when it does
>> that, the __init__ method of the existing instance is called
>> nonetheless, so that the instance is initialized a second time.  For
>> example, please consider the following class (a singleton in this case):
>>
> [snip]
> 
>> How can I prevent __init__ from being called on the already-initialized
>> object?
> 
> 
> Is this an acceptable kludge?
> 
>  >>> class C(object):
> ...  instance=None
> ...  def __new__(cls):
> ...   if C.instance is None:
> ...    print 'creating'
> ...    C.instance = object.__new__(cls)
> ...   else:
> ...    cls.__init__ = lambda self: None
> ...   return cls.instance
> ...  def __init__(self):
> ...   print 'in init'
> ...
>  >>> a = C()
> creating
> in init
>  >>> b = C()
>  >>>
> 
> (Translation: dynamically override now-useless __init__ method.
> But if that works, why do you need __init__ in the first place?)
> 
> -Peter
Or this one: use an alternative constructor:

class C(object):
    instance = None
    @classmethod
    def new(cls, *args, **kw):
      if cls.instance is None:
        print 'Creating instance.'
        cls.instance = object.__new__(cls)
        print 'Created.'
        cls.instance.__init__(*args,**kw)
      return cls.instance
    def __init__(self):
      print 'In init.'

  >>> c = C.new()
Creating instance.
Created.
In init.
  >>> c = C.new()
  >>>

Michael