__init__ and __new__ how to use

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.

Some references:
The description of new style classes goes into some depth about how
instance creation works, including examples of overriding __new__ and
__init__. http://www.python.org/download/releases/2.2/descrintro/