cr.set_source_rgba(0.5, 1.0, 0.0, 1)
cr.arc(widget.get_size()[0] / 2, widget.get_size()[1] / 2, widget.get_size(
    )[0] / 2, 0, pi * 2)
cr.fill()

def main(argc):
    win = gtk.Window()
win.set_decorated(False)
win.set_app_paintable(True)
win.set_size_request(100, 100)
screen = win.get_screen()
rgba = screen.get_rgba_colormap()
win.set_colormap(rgba)
win.connect('expose-event', expose)
win.show()
How to get transparent background in window with PyGTK and PyCairo?