Python gtk3 Drag and drop

[MG&TL]

OK, here's the code I'm trying to run:

#!/usr/bin/python

from gi.repository import Gtk

class UI():
    def __init__(self):
        
        w = Gtk.Window()
        w.set_default_size(600, 400)
        lab = Gtk.Label("drop here!")
        lab.connect("drag-data-received", self.on_drag_data_received)
        w.add(lab)
        w.connect("destroy", Gtk.main_quit)
        w.show_all()
        Gtk.main()

    def on_drag_data_received(widget, drag_context, x, y, data, info, time):
        print("Hello World!")

test = UI() 

However, the window produced is not sensitive to dnd, nor does it print anything if I drop anything on it. What am I doing wrong?