Use __new__ when you need to control the creation of a new instance. Use __init__ when you need to control initialization of a new instance.
__new__ is the first step of instance creation. It's called first, and is responsible for returning a new instance of your class. In contrast, __init__ doesn't return anything; it's only responsible for initializing the instance after it's been created.
In general, you shouldn't need to override __new__ unless you're subclassing an immutable type like str, int, unicode or tuple.
From: http://mail.python.org/pipermail/tutor/2008-April/061426.html
You should consider that what you are trying to do is usually done with a Factory and that's the best way to do it. Using __new__ is not a good clean solution so please consider the usage of a factory. Here you have a good factory example.
转自：http://stackoverflow.com/questions/674304/pythons-use-of-new-and-init