Understanding Python decorators

I've finally come to grasp with Python's decorators, and have managed to create somewhat nicer code. Here's what I had before:

    def on_top(self, event):
        index, item = self.find_shape()
        self.shapes.pop(index)
        self.shapes.append(item)
        self.populate()
        self.list.Select(0)


    def on_bottom(self, event):
        index, item = self.find_shape()
        self.shapes.pop(index)
        self.shapes.insert(0, item)
        self.populate()
        self.list.Select(len(self.shapes) - 1)


    def on_up(self, event):
        index, item = self.find_shape()
        self.shapes.pop(index)
        self.shapes.insert(index + 1, item)
        x = self.list.GetFirstSelected() - 1
        self.populate()
        self.list.Select(x)


    def on_down(self, event):
        index, item = self.find_shape()
        self.shapes.pop(index)
        self.shapes.insert(index - 1, item)
        x = self.list.GetFirstSelected() + 1
        self.populate()
        self.list.Select(x)

As you can see - a lot of repeated code, with only different things happening in the middle of each function. This is ideal for a decorator. I struggled for a while to get it working as a decorator function inside a class, before reading how you define a wrapper function for creating a closure over the decorated function.

My code now reads,

    def move_shape(fn):
        """
        Passes the selected shape and its index to the decorated function, which
        handles moving the shape. function returns the list's index to select
        """
        def wrapper(self, event, index=None, item=None):
            index, item = self.find_shape()
            self.shapes.pop(index)
            x = fn(self, event, index, item)

            self.populate()
            self.list.Select(x)
        return wrapper

    @move_shape
    def on_top(self, event, index=None, item=None):
        self.shapes.append(item)
        return 0

    @move_shape
    def on_bottom(self, event, index=None, item=None):
        self.shapes.insert(0, item)
        return len(self.shapes) - 1

    @move_shape
    def on_up(self, event, index=None, item=None):
        self.shapes.insert(index + 1, item)
        return self.list.GetFirstSelected() - 1

    @move_shape
    def on_down(self, event, index=None, item=None):
        self.shapes.insert(index - 1, item)
        return self.list.GetFirstSelected() + 1

Unfortunately the event parameter is there because these are also wxPython event listeners, and I'd have to create delegate functions just to remove the one parameter, which seems pointless.