Steven Bethard wrote: > Felix Wiemann wrote: > >> How can I prevent __init__ from being called on the >> already-initialized object? > > The short answer: you can't: > http://www.python.org/2.2.3/descrintro.html#__new__
What a pity. By the way, I'm just seeing that the web page says: | If you return an existing object, the constructor call will still call | its __init__ method. If you return an object of a different class, its | __init__ method will be called. However, the latter doesn't seem to be true, or am I missing something? >>> class A(object): ... def __init__(self): ... print 'Init of A.' ... >>> instance = A() Init of A. >>> class B(object): ... def __new__(self): ... return instance ... def __init__(self): ... print 'Init of B.' ... >>> B() # <--------- A's __init__ is *not* called. <__main__.A object at 0x4062424c> >>> instance = object.__new__(B) >>> B() # <--------- B's __init__ is called Init of B. <__main__.B object at 0x406243ec> So there seems to be some type-checking in type.__call__. > Note that in the Singleton example there, subclasses are told to > override init, not __init__ for exactly this reason. I see. > py> class C(object): > ... class __metaclass__(type): > ... def __call__(cls, *args, **kwargs): > ... if cls.instance is None: > ... print 'Creating instance' > ... cls.instance = cls.__new__(cls, *args, **kwargs) > ... print 'Created' > ... cls.instance.__init__(*args, **kwargs) > ... return cls.instance I didn't think of inlining the metaclass; that's really nice. > [...] where all the work is done in the metaclass and you don't even > define __new__. Yeah, that's good. I think I'll go that way. Thanks a lot! -- Felix Wiemann -- http://www.ososo.de/ -- http://mail.python.org/mailman/listinfo/python-list
