[cairo] Alpha channel [pycairo]

[Manuel]

hi all, I've this "program" :

# main.py
# -*- coding: UTF-8 -*-

import pygtk
pygtk.require('2.0')
import gtk, gobject, cairo

# Create a GTK+ widget on which we will draw using Cairo
class Screen(gtk.DrawingArea):

    # Draw in response to an expose-event
    __gsignals__ = { "expose-event": "override" }

    # Handle the expose-event by drawing
    def do_expose_event(self, event):
        cr = self.window.cairo_create()
        self.draw(cr, *self.window.get_size())

    def draw(self, cr, width, height):
        cr.set_tolerance(0.1)
        cr.set_source_rgba(0.0, 0.0, 0.0, 0.5)
        cr.set_operator(cairo.OPERATOR_SOURCE)
        cr.paint()
    
# GTK mumbo-jumbo to show the widget in a window and quit when it's closed
def run(Widget):
    window = gtk.Window(type=gtk.WINDOW_TOPLEVEL)
    window.set_decorated(False)
    window.set_resizable(False)
    window.connect("delete-event", gtk.main_quit)
    widget = Widget()
    
    screen = widget.get_screen()
    color = screen.get_rgba_colormap()
    widget.set_colormap(color)
    
    widget.set_app_paintable(True)
    widget.set_size_request(500, 500)
    widget.show()
    window.add(widget)
    window.present()
    gtk.main()

if __name__ == "__main__":
    run(Screen)

try execute it, the alpha channel doesn't work...
The windows isn't semi-transparent...
cr.set_source_rgba(0.0, 0.0, 0.0, 0.5) doesn't work ??