python: the difference of " __new__" and "__init__"

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.