Subclassing array

[TG]

Hi.

i've already something about inheriting from array a few weeks ago and
had my answer. But again, there is something that I don't understand.
Here is my vector class, which works quite well :

class Vector(array):
    def __new__(cls,length,data=None):
        return super(Vector,cls).__new__(cls,'f')

    def __init__(self,length,data=None):
        if data == None:
            for _ in xrange(length):
                self.append(0.0)
        else:
            for i in xrange(length):
                self.append(data[i])



Now, i want to inherit from this vector class :

class Stimulus(Vector):
    def __init__(self,width,height,label,data=None):
        Vector.__init__(self,width*height,data)
        self.width = width
        self.height = height
        self.label = label

This doesn't seem to work :
>>> s = Stimulus(10,10,"data")
TypeError: __new__() takes at most 3 arguments (4 given)

In order to make it work, it seems that I have to redefine __new__
again, like this.

    def __new__(cls,width,height,label,data=None):
        return super(Stimulus,cls).__new__(cls,width*height)

Why is that ?
When I call Vector.__init__() in Stimulus, doesn't it also call __new__
? I don't understand the detail of callings to __new__ and __init__ in
python inheritance ...