Draw directly in the root window using PyCairo

I want to be able to draw directly in the root window in Python using Cairo. I checked my code drawing in a full screen window, so I know this is not a problem. However, when I try to draw directly in the root window, for example:

root = gtk.gdk.get_default_root_window()
cr = root.cairo_create()
...
# Drawing code here

Nothing happens. The code works fine, but nothing is drawn on the screen. Is there a way to do this, or do I need to overlay a transparent window on top of everything? (The problem with this solution is installation with multiple monitors)