Inheritance and recursion problem

[Spiro]

Hi all,
i'm trying to make some user interface objects in python.

I have classes XWindow and XMainWindow(XWindow) (XMainWindow inherited 
from XWindow)

XWindow have all drawing functionality and Windows[] object which holds 
all child windows.
Function Draw looks like this:
def Draw(self):
	self.Back.blit
	for wnd in self.Windows:
		wnd.Draw(OffsetX, OffsetY)

Now in main module i make objects:
RootWindow=XWindow()
MW=XMainWindow()
RootWindow.Widnows.append(MW)

Now the problem:
When i call RootWindow.Draw() he calls wnd.Draw where wnd is XMainWindow 
from RootWindow's Windows[] collection.
Now, we are in MW's Draw and MW's Windows[] collection should be empty 
but somehow he has itself (well, new instance of XMainWindow) in this 
collection and i got unlimited reference.

...
While writing this post i tried something:
instead of using RootWindow.Windows.append(MW)
i used RootWindow.Windows=[MW] and now it's OK.

I'm happy now, my code works, but i don't know what was happening there: 
why append made new instance of object instead of passing existing object.