why it doesn't use __init__ but use __new__ here?

I have beeing studying __new__ recently, and I read lots of codes where it use __new__ instead of __init__, some times I think both works, but I don't know why the original author use __new__, to be specific, can someone explain why below codes use __new__ instead of __init__?

case I from here

    class MiniSubtest(object):

    def __new__(cls, *args, **kargs):
        self = super(MiniSubtest, cls).__new__(cls)
        ret = None
        if args is None:
            args = []
        try:
            ret = self.test(*args, **kargs)
        finally:
            if hasattr(self, "clean"):
                self.clean()
        return ret