I am trying to position a dialogbox relative to the main GIMP window (using Python-fu)

1. I load a file in GIMP as follows

first_file = os.path.join(PathToProcess, filename)
img = pdb.gimp_file_load(first_file,first_file)

2. I try to get the position of the file as follows

wdw = gtk.Window(gtk.WINDOW_TOPLEVEL)
xm,ym = wdw.get_position()

xm, ym returns as 0,0.

I have tried different ways to reference the main GIMP window, but it does not seem to work.
How should I get the position of the main GIMP window (with my file opened) on the deskto ?